Similar to OMG (Oh my god!), but ending with a fashionable "osh" to imply "Oh my gosh!", which isn't "Oh my god!", which is important, if importance was the issue. Which it's not.
The term "omgwthigo" is an acronym that stands for "Oh my goshwhat the heck is going on?!" normally used when the person either truly is clueless on what is happening (usually wherever the other person is communicationg from) or as an expression while in a confusing online conversation, mostly with more than two people involved.
The longer version is more explicit, and self explanatory on what it is
"omfgwtfhifgo"
User 1 says: There is no way that this ishappening User 2 says: I know srsly this is mssd up
Used 3 says: Unbelievable User 1 says: Yeah
User 4 says: omgwthigo