It stands for 'One True Pairing' and is used in fandoms, I would say that it is better explained as 'Oh the pain' but that's not the real definition. There are some cases where one person in your OTP dies, basically you get a broken ship T-T
Person: So Marco x Jean is your OTP?
Marco x Jean fangirl shipper: YES, BUT IT'S A BROKEN CANON
On the potty
Sorry I can’t talk right now, I’m OTP and you probably don’t want to hear that.