Great topic, Elm!
Ok, maybe 'Oy!' does need explanation. I was the first one to use it, and it's basically an exclamation of greeting, surprise, or agreement.
Yo! = a reverse of 'Oy!' It is said as a greeting and was first used by @Iliara_Mustardtop.
Variations of oy and yo include OYOYOY and YOYOYO.
Oya! - coined by @Aislinn_Mollisong1. It is the official greeting of Leifer, a world in one of her books.
Meh-heh?! = first used by @Rachel_Blackruin. It most often is an exclamation of astonishment or surprise.
Teh = first used by @Hanna_Brassheart, I believe. It developed into 'teh-heh', which is a variation of 'meh-heh'.
Yepperdoodles = first used by @Xonos_Darkgrate1 as a variation of the word 'yes'. Some people also say ''yepper'.
Zooterkins - first used by Hanna, most often as an exclamation of surprise or disbelief.
Oofta/Ooftah! - first used by @May_Otterview as a variation of 'oof'.
Smae = random gibberish word created when someone misspelled the word 'same'.
Supermoderators - the @moderators
Feel free to correct these definitions, if I've made a mistake anywhere.