I've been working with Oxygen for the past few months, building a fairly complex website for a church. It took a little digging to figure out the homepage slider and how it works w/ the posts, and some trial and error to figure out some other details -- but I am a WordPress newbie (this was my fifth or sixth site), so more experienced developers might find it a cakewalk.
Anyway, I do love the theme. Using custom menus and the catlist plug-in makes it even more flexible. I've had little or no need to dive into the css or php files. I've found everything to work "as advertised," and wouldn't hesitate to use this theme again in the future.