The Nature of Transition Circumstellar Disks I. The Ophiuchus Molecular Cloud ⋆ Lucas A. Cieza 1 , $^{2}$, Matthias R. Schreiber$^{3}$, Gisela A. Romero3 , $^{4}$, Marcelo D. Mora$^{3}$, Bruno Merin $^{5}$, Jonathan J. Swift$^{1}$, Mariana Orellana3 , $^{4}$, Jonathan P. Williams$^{1}$, Paul M. Harvey$^{6}$, Neal J. Evans II 6 Received ; accepted To appear in ApJ ⋆ Based in part on observations made with ESO telescopes at Paranal Observatory, under ESO program 083.C-0459(A). $^{1}$Institute for Astronomy, University of Hawaii at Manoa, Honolulu, HI 96822. $^{2}$Spitzer Fellow, lcieza@ifa.hawaii.edu $^{3}$Departamento de Fisica y Astronomia, Universidad de Valpara´ıso, Valpara´ıso, Chile $^{4}$Facultad de Ciencias Astron´ omicas y Geof´ısicas, UNLP, La Plata, Argentina $^{5}$European Space Agency (ESAC), Villanueva de la Canada, Madrid, Spain $^{6}$Department of Astronomy, University of Texas at Austin, Austin, TX 78712 arXiv:1001.4825v1 [astro-ph.GA] 26 Jan 2010