Apart from 'One True Paring',
OTP is New Zealand slang for 'On the piss '.
'Piss' can be slang for alcohol, so On the Piss means to be drinking alcohol (usually a lot) or used when someone is talking about drinking alcohol in the future.
(Facebook status):
Getting OTP this weekend!
or
*Jacky* and I are OTP! Great party!
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