A really enjoyable first episode, I had a really fun time while I was watching the stream.
While many are comparing it with Den-O I believe it's only in terms of absurd comedy (are we going to have a shot of Eiji in his pantsu each episode?), I don't think Den-O had the same levels of creepyness we've already had in OOO's first episode (the "Happy Birthday" alone is...).
I've liked the opening too, although W's is still better in my opinion. I don't mind the TaToBa tribal chant-thing but the visuals of the henshin could have been better. The fight was interesting and showed how OOO's system works, I like it even if it felt too similar to W's Gaia Memory changes. The "medals = kaijin's blood" is like taken from a videogame... but it fits well. Cliffhanger reminded me a bit to Kiva's first episode cliffhanger.
Really looking forward for episode 2, next Sunday can't come soon enough.