There were a number of "gun" mods for oblivion but all of them offered little more than very elaborate staff weapons. Looking at both the Oblivion Construction Set and GECK, it seems to me that guns are more probably based on the ranged weapon set (with projectile drop ignored and speed dramatically increased) rather than magic. Unfortunately, it seems nobody has tried making a gun from the ranged set, and it may be the result of some critical portion being hard coded (I have seen no reference that would allow one to alter the time it takes to "draw" the bow - a requirement for a remotely realistic firearm).
The similarities between the two games are fairly obvious however. The drugs in Fallout are literally the same class of item as the potions in Oblivion. New models and the inability (without mods) to make your own morph...I mean med-x disguise this to a degree. Likewise, perks apply effects in the same was as magic, and the same goes for item effects. Weapons like the flamethrower and rocket launcher actually are apparently governed by similar rules to the casting system in Oblivion. As a result, I have seen many, many mods where content from Oblivion has been placed in Fallout, but I have never seen the reverse done. I do not know if this is the result of nothing more than a lack of interest of if it occured because of some sort of technical problem. Replacing various NPC's with thsoe from Fallout would likely be easy enough, but the changes would be purely visual. That said, I'm fairly certain putting the Deathclaw into Oblivion is something that OUGHT to be done. I may even try my hand at it, but I'm not sure if I'd go beyond simply doing a model sweap for the triceritops looking oblivion monster (since it has more or less the same AI it would seem).
If the projectile issue could be cracked, and if a few issues with the Oblivion magic system could be sorted out, I'd say it's perfectly possible to put a bit of wasteland flavor into Tameriel. I'm not certain the result would be terribly fun however, and it may even break the game entirely. At the very least stats across the board would HAVE to be adjusted. As it stood in Fallout 3, the melee sytem was identical to the one in Oblivion but there was no compelling reason (beyond the fact that it was harder) to use even the most powerful melee weapons in Fallout.
The more I think about the AI angle, the more I realize that it may be inconsequential as the core logic that drives the various things appear to be the same. Bloatflies and gun toting raiders stand in place launching projectiles and occasionally move but rarely seem to intentially take cover. Melee creatures and characters simply make a bee line for the character and start swiging with their best attack with the occasional fast attack to spice things up. This would mean adding melee raiders and super mutants would be no more difficult than adding them to the leveled lists along with their weapons (which are easy enough to import and assign into broad categories of blade and blunt). That more or less leaves the projectile problem, which I've never seen cracked in spite of a number of notable attempts.