online - that which is not offline; the condition of being connected to or on the internet. Usage example: "I come online first thing in the morning", "I am online 24/7", "I'm not on with her in RL. We met online. She lives on the other side of the planet".
OIC - oh I see.
OMG - oh my god.
OTP - on the phone. An explanation of the chatter's temporary absence from the chatroom conversation.