OTP means On The Prowl!!! as in "feeling frisky" like a cat who is out on the hunt to get her prey! It can be used in a sexy way (like "she is looking to get some tonight"), or in a joking way about being overly desperate about being on the hunt for anyone.
When she saw her friend(who was wearing a short, tight red dress and high heels, looking very sexy), she exclaimed, "Oooh! I see you are OTP tonight!"