A person who is obviously a Jew. OJ's usually are recognizable by their name. To spot an OJ, look at their name. If it sounds Jewish, it probably is.
Be aware that not all people who have Jewish-sounding names are actually Jewish, so don't write in another OJ in your Obvious Jew Scouting notebook unless you're sure they really are a genuine Jew.
Person 1: "Hey, is Ben an Obvious Jew?"
Person 2: "Who, Ben Silverberg? You betcha"
Person 1: "Aaron Goldman is an OJ, right?"
Person 2: "You got that right"