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1.
OMU
Oh My Unicorn!
Based of of the cult of unicornism.
OMU I can't believe that Dave has such a small 'unicorn horn'
by Unicorn Gal November 21, 2010
 
2.
OMU
Occasional-Marijuana-User:
This guy is friends with many pot heads and loves being around them when they are high but he only smokes with them ever once in a while. He is often sitting back and observing their idiotic decisions, He hates being called a pothead and denies that he smokes at all costs.
Pothead 1: bro this is somr good ass weed, take a hit!

Pothead 2: Hell yeah ill have a hit. What about you Dave?

Dave: I'll pass. Im good for now.

Pothead 1: Nah hes a fuckin omu, hes probably talking shit about us righy now.
by kdspencer18 April 20, 2013
 
3.
OMU
meaning 'oh my Urie', referring to Brendon Urie of Panic(!) at the disco. Commonly used by my friends, because they get annoyed that i say oh my Stump a lot. used as a replacement of god/gosh in the phrase 'oh my god' Originated in Pennsylvania, to my beliefs. can be alternated to omfu, meaning oh my fucking/freaking Urie.
One of My Friends:"OMU, Brendon Urie's voice sets my pants on fire! Epically when he hits the high notes! *giggle* <3"
Me: "I know!!!! Wait, did you just say OMU?"
by I'mMeSmarty July 21, 2009