OTP is a fandom/fanfiction term for "The One True Pairing", or One True Partnership if you prefer, of two characters who are considered to be 'made for each other' or ideal (romantic) partners. There are variants such as OT3 for One True Threesome.
The term OTP is often used sarcastically or jokingly - in fact, around Profusion it's more or less always sarcastic or joking.