Short for On The Piss, slang for the act of getting drunk.
Also, OTPTN (On The Piss Tonight)
and SOTP (So On The Piss)
and lastly, NOTP (Not On The Piss)
"Hey bro, are you OTPTN?"
, I'm getting fucking rebecca blacked."
"NOTP, I have to study"
HSC IS OTP!