Basically, Oblivion is about trying to find the lost heir to the throne and helping him close the hellish portals to Oblivion (Oblivion resembles hell). Most of the game is okay for 10+, or at least a mature 8 year old. Gore is very minimal. There is one place in the game where this woman likes to torture people in a chamber with torture tools but they're not very gory tools. The only disturbing thing here is that the woman likes to torture a specific race for no reason. It's one quest, though, and not very easy to stumble across.
Oblivion is not dark and there is blood but it's only a little bit. Sometimes you'll see (extremely unrealistic) blood splatters. It's an older game than Skyrim so it doesn't look real. The only reason this game was rated M was because some weirdo made a mod that makes all the women in the game topless but that's only on PC. If your child is getting a 360/PS3 version, they'll be fine.
There is drinking but I've only ever heard someone casually mention someone being drunk in the street. There is mead, ale, and wine and some wineries but all the drinks do is lower your health points and stamina points.
In terms of sexual stuff, I've never heard anything EXCEPT I've read an in-game that references some obscene things (only references, it never actually directly says it. All it is, is a lot of innuendo.). If there is anything else I haven't heard, it's most likely just innuendo and doesn't say directly what happens.
The swearing is not too positive, mostly 'damn', 'ass', 'what the hell', and 'bastard'. Damn isn't frequent but is probably the most common word. I've only ever heard ass once. What the hell is said a few times. Bastard is infrequent but still noticeable.
Despite some bad messages (i.e. a guild that pays you to kill people (mostly by knife or poison), a group that steals for a living) Oblivion is totally fine. If you are caught trespassing or assaulting or stealing or killing anybody, you will be either fined or imprisoned.
The last thing is there is a very addictive, cocaine-like drug that you can drink called 'skooma'. It's barely mentioned, though.
Overrall, it's not an extremely bad game but does have some bad spots, but then again, all games do.