Here's Devil & Devil:
http://www.mangatraders.com/manga/series/132

I don't know what the fuck Os-Tan is, but doing a search it seems like it is some hentai shit. Could care less about helping you with that.

Worth noting that there's another Devil & Devil around that is yaoi. If that's what you're looking for, once again, I could care less about helping you with that.