I had to look it up. Captain Action was the second "action figure," designed like the first action figure, G.I. Joe, by Stan Weston. Captain Action's gimmick was more accessories. He could change into all sorts of other characters, including Superman, Batman, Spiderman, etc. etc.
He had a sidekick Action Boy, whose Onesied up logo this might be.
This description of Captain Action from Wikipedia makes me wonder how well-suited he is for infant clothing: "The figure itself had a rather sad and worried expression, a strange shaped head (so the masks of the various heroes would better stay in place over it) and a more detailed musculature than G.I. Joe's."
Or maybe it's enough to be the best/only Onesies for sale at Comic-Con.
Captain Action [wikipedia]
hmm. that doesn't look like a Onesie (tm) to me.