Noun. acronym for the "Open for Business" sign - as in, eagerly seeking a relationship with the opposite sex. Most typical of single females in their mid to late 20's when their apparent lack of wanting to be alone becomes apparent to the untrained eye.
Dave: "That's weird, Eve just asked if I want to go to lunch on Wednesday. Didn't she just get out of a relationship with Adam?"
Russ: "About a month ago, and I had lunch with her last wednesday. Her OFB is in the window."