Not sure. The landscape is drab and the atmosphere grim. There is horror - well, sort of, more a case of lots of gore and enemies that fulfill certain horror staples. However, you are armed, and unlike survival horror you can make sure you are well armed before entering any location.
I think a better way of getting a feel for its vibe is to understand it blends a classic, 50s movie look and feel for surviving in the aftermath of nuclear war with a more modern, very Mad Max (minus the cars) feel with brutal tribes, bloody death and murder being in major supply.
Obviously the core engine and feel is very like Oblivion, the main difference I felt was that in Fallout 3 you pick what to level up in normal RPG fashion vs automatically leveling up based on what you're doing - as was the case in Oblivion. Another big difference is VATS. With that you really move to a more pure RPG approach were combat is stat based and automatic vs you moving/shooting in real time. In fact VATS may be the main difference now that I think about it.
To me they felt like very different games because of setting, plot, approach, etc. - however I can see that others might find them to be essentially the same game with different textures.
One thing I'd note too, although you can have a TPS view if you prefer, it's as poor as Oblivion. Fallout 3 for me plays and looks best as a FPS in terms of core movement.
So I'd read up on VATS and the leveling system, as they make Fallout 3 much more of an RPG than Oblivion in my eyes, maybe take in a gameplay video or two to judge how you like the feel of the game, then go from there.
Try to be reasonable... its easier than you think...
