Name the total number of opponets venue for oklahoma
CREATE TABLE "record_vs_big_12_opponents" (
  "missouri_vs" text,
  "overall_record" text,
  "at_columbia" text,
  "at_opponents_venue" text,
  "at_neutral_site" text,
  "last_5_meetings" text,
  "last_10_meetings" text,
  "current_streak" text,
  "during_time_in_big_12" text
);
SELECT COUNT("at_opponents_venue") FROM "record_vs_big_12_opponents" WHERE "missouri_vs"='Oklahoma';