Acronym for "on the internet". Essentially, the opposite of "irl" (in real life.)
You don't understand me very well OTI: next time, call me.
Over The Internet
Dude_1: "Hey I went to a cool party last night."
Dude_2: "Cool, got any pictures?"
Dude_1: "Yeah, let me jsut email them to you OTI ;)"