Fallout 3 uses many aspects of the Oblivion system, but to call it "Oblivion with guns" is overly simplistic, because guns are only a small part of the changes to the combat system, which is itself only one of several changes.
The leveling system is almost directly copied from previous Fallout games, which works perfectly. In comparison to Oblivion, this means that you worry about experience rather than using your skills frequently. One thing that they did really well was to make sure than a significant portion of your experience comes from non-combat things. 18xp is the most I've gotten so far from a kill (most things have been 3-6), while hacking an average computer terminal yields 56xp. You also gain XP from finding new locations.
There is no magic - that might seem intuitive, but it also an important difference between Fallout and Oblivion.
NPC dialogue is vastly improved from Oblivion, although it doesn't seem quite up to par with previous Fallout games. There are a lot more generic NPC's with no dialogue, but their existence does not detract (in comparison to previous Fallout games) from the frequency of unique NPC's, instead they make the world seem much more realistically sized. In about ten hours of play so far, I've only once encountered a time when I wanted a dialogue option that wasn't available.
The house-owning mechanic has come over from Oblivion, with a few tweaks, and I find it a welcome addition to Fallout, primarily as a source of storage space.
Combat is the biggest change, because it's not like Oblivion or previous Fallouts. It's real-time, like Oblivion, but takes place at much greater range. The environment also matters a whole lot more, providing cover and occasionally exploding (as I discovered the hard way when I tried taking cover in the middle of a road full of cars. It was the first time I had seen my character lose all four limbs in a death sequence). Grenades and mines change the dynamics almost as much as guns, especially since they do a realistic amount of damage in comparison to guns (i.e. A LOT). V.A.T.S. is incredibly useful for someone like me who is bad at FPS, and compensates for that lack of ability nicely. The fact that it's limited by action points means that people who are good at FPS will still benefit from their skills. It lets you shoot grenades out of the air. Other people can probably tell you a lot more more about the combat system though - I'm playing a scrawny scientist, so most of my combat experience is being shot at rather than shooting back.
My sense of it is that it takes the best elements of both Oblivion and the Fallout series and combines them. It will feel like Oblivion if you've played that before, because the interface is similar. If you've played a Fallout game before, this will also feel similar because the look and feel of the world and NPCs are the same. I've played both, and to me it honestly feels more like Fallout with the Oblivion interface rather than Oblivion with guns.