I have yet to finish OOO, but my two big problems with the show are:
1) It's a poor man's Den-O. Whereas Den-O, for all its flaws could at times be genuinely funny, OOO feels like a cheap attempt to replicate Den-O and failing miserably. Comedy is shoehorned in at the most inappropriate moments, and jazzy music does not match the fight scenes at all. Toshihiko Sahashi, this guy wasn't. OOO's lyrical songs could be ok, but the BGM was terrible.
2) Kougami and Maki. I don't mind Kougami's running "birthday" gag and Maki's doll, but they both act like they just escaped a mental asylum. And especially when it was implied that Kougami was being funded by Foundation X in the W finale, why is it the heroes treat him so casually? Why on Earth would you decide to shoot a random 40th anniversary KR movie in his building when earlier Ankh was ready to tell him off for the deal he forced them into to take his Cell Medals??
As for Maki's doll, they should have been more low-key with it. Have Maki talk "to him" in his lab when he is talking out loud by himself or to a guest. Maybe sort of imply that the doll is "watching" people without doing it in a cartoonish or "for laughs" way. Don't carry him around on your arm everywhere, dress him up in outfits, have him "drive" your car, and act like a crazed monkey if anyone knocked him over. It's not creepy. Nor is it funny. It's stupid.
Then there is Goto, who doesn't look the part of an ex-cop at all, Eiji's general cluelessness and having to be lead around by the nose everywhere, the Greeds disappointingly being watered down Grongis (Gamel especially was so misused, he really could have been something more intimidating. Not an overgrown child who has no idea about anything but somehow got so much love from the fans, at least on this board), and some other things I can't recall off the top of my head right now.
One thing I will say though is I really like the Yummy designs. Their human faces particularly have a creepy feel. It would have worked much better in another show.
And I sort of wish we saw more of the Ride Vender Platoon guys from the first episode. Have them act as spies, reconnaissance, and Cell Medal collectors instead of the Candroids. Maybe they're not as marketable, but it would help show the reach and power of the Kougami Foundation behind its guise as a simple biotech organization and provide an opportunity for grunt battles besides the Waste Yummies.
I'm guessing the ZECTrooper figures from Kabuto didn't do that well.