TEXT: Author’s Notes on the Rules
  Writing, editing, layout: © 2018 Gavin Norman. Header art: © 2017 Andrew Walter. 
  Old-School Essentials is a trademark of Necrotic Gnome.
  Old-School Essentials is the product of 
  over two years of poring over venerable 
  tomes, discussing rules minutiae with 
  the elite of online RPG geeks, laborious 
  writing, and painstaking layout work.
  The result—in the form of the Core Rules 
  and Classic Fantasy books—is a game 
  100% compatible with the classic Basic/
  Expert rules from 1981. However, no 
  game is perfect, and Old-School Essen-
  tials attempts to fix some of the most 
  irksome errors in the Basic/Expert rules.
  This document describes the tweaks that 
  I made, for those with a passion for rules 
  archaeology.
  Ambiguities and Omissions
  The original Basic/Expert rules are not 
  devoid of ambiguities: some rules are 
  incomplete; some rules are described in a 
  way that could be interpreted in multiple 
  different ways; some rules even contain 
  contradictions.
  With the aim of increasing the playability 
  of the game, I decided to iron out such 
  ambiguities in the rules. In doing so, I 
  was careful to cleave as closely as possible 
  to what I interpreted as the intent of the 
  original rules. Other interpretations of 
  the rules—and other resolutions of the 
  areas of contradiction—are certainly 
  possible, though.
  Expansions of Scope
  I have expanded the scope of a small 
  number of rules, applying them to more 
  general situations than are explicitly 
  allowed by the original rules.
  Additional Rules
  I decided (after much soul-searching!) 
  to include a single, additional rules 
  option to the game: ascending Armour 
  Class. The rationale for this is discussed 
  overleaf.
  
  Core Rules
  Ambiguities and Omissions
  Ability Checks
  The original rules are somewhat ambig-
  uous on whether ability checks are an 
  optional rule or not. They are specified 
  as an optional rule in the original Expert 
  rules, but are recommended for use in 
  climbing. The original Basic rules do not 
  specify ability checks as an optional rule. 
  On balance, I decided against specifying 
  ability checks as optional. Their appli-
  cation is, of course, entirely up to the 
  referee.
  Encumbrance
  The original Basic rules contain ambigu-
  ities relating to the encumbrance system. 
  I have attempted to resolve these ambigu-
  ities in a sensible and consistent way, but 
  other interpretations of the original rules 
  are possible.
  Retainers
  The original Basic rules stipulate that 
  retainers only receive half the experi-
  ence points due to them. There are two 
  possible interpretations of this. It may 
  be that retainers receive 1 XP per gp of 
  treasure they earn (the same as any other 
  adventurer), but that there is an unspo-
  ken assumption that retainers receive a 
  half share of treasure. It may, alternative-
  ly, mean that retainers only receive ½ XP 
  per gp of treasure. The actual intention 
  of the Basic rules is unclear; I chose to 
  favour the latter interpretation.
  Traps
  The distinction between room traps and 
  treasure traps is implicit in the original 
  Basic rules, but is never clearly pointed 
  out. In my opinion, the rules presented 
  in Old-School Essentials are the clearest 
  way to resolve the ambiguity between the 
  general adventurer ability to find traps 
  (i.e. room traps) and the traditional thief 
  class’ ability to find small traps (i.e. treas-
  ure traps). The clarification presented 
  is far from the only possible resolution, 
  though.
  Movement in Combat
  The original Basic rules do not make 
  it clear whether characters may run in 
  combat (using the rules for evasion and 
  pursuit in the dungeon). However, a later 
  edition of the game makes it clear that 
  this is not possible, so I have followed 
  that clarification in Old-School Essentials.
  Morale
  The original rules are ambiguous as to 
  whether a morale check is triggered the 
  first time a member of the monsters’ side 
  in combat is killed, or whether a check 
  is triggered the first time a member of 
  either side is killed. I have clarified this to 
  what I feel is the most sensible interpreta-
  tion: monsters check morale the first time 
  one of their own number is killed.
  Treasure Type Average Values
  According to my re-calculation of these 
  averages, there were errors in the original 
  Basic rules. The average value for treasure 
  type M, in particular, was way off.
  Archaeological Notes
  
  Expansions of Scope
  Hirelings
  The original rules do not use this term 
  (or indeed any term) to refer to non-ad-
  venturing NPCs that may be hired by 
  PCs. The term hireling is introduced to 
  conveniently refer to all types of such 
  characters, including mercenaries and 
  specialists.
  Subduing
  The rule for subduing is mentioned in the 
  original rules in the context of dragons. 
  I chose to place this rule in the combat 
  section in Core Rules, as it could easily 
  apply to other types of creatures (includ-
  ing humanoids).
  Vehicles
  The rules listed in this section original-
  ly applied only to water vessels. I have 
  broadened their scope to encompass all 
  types of vehicles.
  Types of Magic
  In the original rules, the two types of 
  spells are simply described as “cleric 
  spells” and “magic-user and elf spells”. In 
  order to expand the scope to potentially 
  include other character classes, I use the 
  terms “divine magic” and “arcane magic”.
  Additional Rules
  Ascending Armour Class
  The optional rule for ascending Armour 
  Class is not present in the original Basic/
  Expert rules. There are two strong rea-
  sons for integrating this option into the 
  core rules:
  1. Use of ascending AC is probably the 
  most common house rule applied by 
  modern players to the classic Basic/
  Expert rules framework. (The sec-
  ond most common probably being 
  separating character race and class, in 
  the vein of the traditional Advanced 
  rules.)
  2. Armour Class is a fundamental game 
  mechanic, and, as such, is referred to 
  throughout all rules modules (for ex-
  ample, in equipment lists, many spells, 
  all monster descriptions, etc.). Apply-
  ing house rules to such core mechan-
  ics of the game is awkward, entailing 
  either the significant adaptation effort 
  of all materials in use, or an on-the-fly 
  conversion whenever the mechanic 
  comes up. Neither is trivial. (In con-
  trast, for example, optional rules for 
  creating characters with separate race 
  and class can be easily addressed in a 
  rules supplement, without affecting 
  the core rules of the game.)
  On balance, I felt it was better to include 
  ascending AC as an option for those 
  groups that prefer to use it. Groups that 
  prefer the traditional AC mechanic may 
  simply ignore the optional rule for AAC.
  
  Classic Fantasy: 
  Genre Rules
  Ambiguities and Omissions
  Halfling
  The original Basic rules include a hal-
  fling tongue in the list of languages that 
  characters may learn, but do not specify 
  whether halfling adventurers speak this 
  language. I chose to make the obvious 
  clarification that halfling characters do 
  speak this language, in the same way that 
  dwarf and elf adventurers speak their 
  racial languages.
  Fighter Extra Attacks at High Levels
  The suggestions for high-level play in 
  the original Expert rules are ambiguous 
  about the level at which fighters gain 
  extra attacks per round. 
  Weapons
  The weights of the following weapons are 
  not specified in the original Basic/Expert 
  rules, and have been extrapolated from 
  similar items: javelin, lance, sling, staff.
  Weapon Combat Stats
  The range of the javelin is not specified 
  in the original Expert rules, and has been 
  extrapolated from similar items.
  Water Vessels
  The exact capabilities of the different 
  types of ship in the original Expert rules 
  are somewhat contradictory. The details 
  presented in Old-School Essentials re-
  solve these contradictions in one possi-
  ble way, while sticking as closely to the 
  original rules as possible.
  Unseaworthy Vessels
  The draft of canoes and rafts were not 
  defined in the traditional Expert rules. 
  The listed values are extrapolated from 
  similar vessels.
  Mercenaries
  The original rules are unclear about what 
  type of horses mounted mercenaries ride.
  Specialists
  The wages of assistant armourers are not 
  defined in the traditional Expert rules. 
  The listed value is extrapolated from 
  similar specialists.
  Strongholds and Domains
  The original Expert rules for calculating 
  the cost of towers are highly ambiguous. 
  The rule presented in Old-School Essen-
  tials is only one possible resolution of 
  this ambiguity.
  Expansions of Scope
  Land Vehicles
  AC and hull point ratings are listed for 
  carts and wagons. These were not speci-
  fied in the original Expert rules.
  Classic Fantasy: 
  Cleric and Magic-
  User Spells
  Ambiguities and Omissions
  Detect Invisible
  I added a description for this famously 
  absent spell, drawing the details from a 
  later edition of the game.
  
  Classic Fantasy: 
  Monsters
  Ambiguities and Omissions
  Bull Shark
  The original rules state that bull sharks 
  use a ram attack to stun prey, but do not 
  give mechanics for it. The rule in Old-
  School Essentials is derived from a later 
  edition of the game.
  Dervish
  The suggested equipment is adapted from 
  the 1979 Advanced rules.
  Giant Bass
  In the original Expert rules, the giant bass 
  is only mentioned inside the nixie mon-
  ster description. In Old-School Essentials, 
  I have adapted it to a full monster, refer-
  ring to a later edition of the game for the 
  descriptive details.
  Ochre Jelly
  The description of the ochre jelly in the 
  original Basic rules is ambiguous on 
  several points. I have attempted to re-
  solve these ambiguities in a sensible and 
  consistent way, based on the mechanics 
  used for similar monsters, but other 
  interpretations of the original monster 
  are possible.
  Sea Dragon
  The sleeping percentage is extrapolated 
  from dragons of like HD.
  Sea Snake
  The original rules do not specify the ef-
  fects of sea snakes’ poison. This has been 
  clarified to be death one turn after the 
  poison’s effects are noticed.
  Classic Fantasy: 
  Treasures
  Ambiguities and Omissions
  Gauntlets of Ogre Power
  The description of this item in the origi-
  nal Basic rules is somewhat ambiguous. 
  It’s not clear that the bonuses only apply 
  to melee attacks. The listed unarmed 
  attack damage also contradicts what is 
  listed in the Expert rules.
  Potion of Growth
  The original Basic rules do not note by 
  how much the character’s STR increases, 
  and are not explicit that only damage of 
  melee attacks is increased.
  Staff of Wizardry
  The exact nature of this staff’s whirlwind 
  power is not clear in the original Expert 
  rules. I have specified a duration of 3 
  rounds, based on the pillar of flame effect 
  of the efreeti monster.
  Warhammer +3, Dwarven Thrower
  The range of this weapon is not specified 
  in the original Expert rules, and was tak-
  en from a later edition of the game.
  
SOURCE: textdata/thevault/Old-School Essentials BX Essentials/Old-School Essentials Authors Notes on the Rules.pdf