In terms of basic gameplay Bioshock and Oblivion have certain elements in common. Both are free-roaming, first-person role players with several quests to do and places to explore. You can choose to ignore the main story line and simply scope out the environment. There are however several differences. The most obvious is art design, in that whilst Oblivion is set in a traditional Tolkien-esque fantasy land of orcs, trolls, elves, magic etc. Bioshock is more of a steampunk affair with a storyline revolving around a technological city corrupted by power and biogenetics. It also offers a slightly more linear story path since you have to do certain tasks in a certain order to complete the game. The gameplay itself also differs in that Bioshock has you using guns as your main weapon, with different customisable options. There's also the idea of plasmids which work sort of like spells in Oblivion, enhancing your character's abilities significantly. Both are amazing and classic games in their own right for different reasons and both are (or rather will be) worth owning.