OMT is a variation of OMG meaning "Oh My Toph".
It was created when a fan of the hit show
Avatar: The Last Airbender no longer found "OMG", or "Oh My God", a suitable acronym to describe events occurring in the show, half because the show required a more fitting letter and half because the fan didn't like using the Lord's name in vain.
And because Toph is one of the most kickass characters, the G was substituted with a T to represent the general awesomeness of Toph.
The term was first used in conversation between two fan fiction writers, but
spread once the duo began a livejournal community called CAPSLOCK_TOKKA. Afterwards, as the term became more
well-known, it
spread to various other websited related to
Avatar, including ASN and KF.net.
Avatard: Did you see the new episode of
Avatar that was released in
Canada?
Tokkaneer: No, was it any good?
Avatard:
Yeah, apparently Sokka and Toph got busy; good times were had by all, at least until Katara walked in and saw them.
Tokkaneer: OMT, that's awesome!