"OK Beast" was created by an anonymous user on the /b/ forum on 4Chan. People were unsure if OP wanted to actually start a meme or wanted to create bait. Either way, the meme started to slowly gain traction.
The meme is used to signify something savage happen in an awkward situation, or some retard doing retarded things. This meme has many uses. OK Beast can also be used for clutch moments in an ironic situation (or when something happens that did not go intending but turned out clutch)

Examples:
Man is in a 1v5 suitation in Rainbow Six Siege. Man says "Ill 1v5 pistol kill only". The whole chat should respond "OK Beast"
If the man starts to actually kill a lot of people, you can repeat the "OK Beast" phrase
Even if the man doesn't clutch but makes a Rambo type effort, you are allowed to respond "OK Beast"

Man kills a fully geared guy in Rust with his rock
"OK BEAST"

Man robs his own store by accident
"OK BEAST"

Man accidentally shoots himself
OK BEAST

Another example comes from Reddit user Eddcus

>be autist
>hatch master plan
>decide to blatantly force a meme so stupid that anons use it ironically to make fun of me
>okbeast.jpg
>attach a common pepe with shit tier font for extra bait capabilities
>anons take the bait
>meme being used to mock me, or so they think
>actually just mind gamed them into repeating a retarded phrase
>some normie screenshots it and posts it to Reddit

Reddit User Lucalu99

>Likes this stupid meme
>Ok Beast

>Exist
>Ok Beast

This meme has endless uses