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!