Standing for "Offer To Adopt", it's a term mostly used on art websites such as DeviantArt for adoptables. You can buy them winning a bid or autobuying them.
You're at a bar and you see some fine early 30's to 40+ women. These types of women look out of place in a college bar. They are OTA. "Dude, check out those females over there" "Nice, but a little OTA"