Ahead of its broadcast premiere,
according to this post the first six episodes of
OK K.O.!: Let's Be Heroes is now available for viewing on the Cartoon Network mobile app and on VOD. Can confirm that all six are currently up on the Spectrum TV app.
I watched all the released episodes and really liked it! The animation is all over the place in a good way. They play incredibly fast and loose with how characters look and move, to the point where the number of fingers they have change from moment to moment. Heck, some episodes look totally different from one another. Episode 3's animation in particular is very out there.
Don't go in expecting another deep lore show: there's definitely an established world in place, but it's all for fun and laughs.
EDIT: Rebecca Sugar sings/wrote the ending credits and she has a nice, cute lyric about animation in general: "Every second that you see is/twenty-four connected pieces".