"ossu" is a term used on the irc channel #GAYM on the ChatSpike network to replace osu (or osu!), which is a rhythm game where you move and click your mouse to the beat.
The reason this term is used is because the bot in #GAYM will remove your ability to talk in the channel if you say "osu."
<Guy1> Let's play some osu!!
*** GAYM sets channel #GAYM mode -v Guy1
<Guy2> ossu sCK
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#GAYM is an irc channel on the ChatSpike network where you can play simple text/symbol-based games through a bot called GAYM. The channel used to be called #GAY but was changed for... obvious reasons. The bot also has several quirks, such as if you say "gay" it will respond with "GAY" in all caps.
<Person> this channel is really gay
<GAYM> GAY
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