Well, Oblivion has more in common with FPSs than RPGs. From a graphical standpoint, Oblivion is a better looking than NWN1 or 2 easily. Its UI is very console based, having been released on the 360 before the PC version. There are UI mods available for it to address that deficiency though. Other mods add better looking armor, weapons, clothing, terrain, skins, undergarments, etc. Since the game also scales to your character's level, you will always be challenged in fights. Unfortunately, this means that if you are level 30 and taking a stroll through the woods near the Imperial city and a wolf attacks you, you'll experience the same level of fight at level 30 as you did at level 1. Combat in Oblivion is also very simple, far from the involvement of the NWN series and no where near the strategic level of the BG and IWD games. In Oblivion, simply click to swing your weapon at the target until it dies. Or, if a castor class, hit your cast button at the target until it dies. Very very simple.
Oblivion falls short on story and plot, falls flat on its face in the dialogue and character development departments, and story/plot oriented mods don't exist. You are the only character you can control directly in Oblivion, though there are some companion mods, that all are better described as henchmen than party members. You have no dialogue to speak. You approach NPCs, initiate 'conversation', and you click on single words to get more information from the NPCs. Words include 'Gossip', 'Location', and quest inquiries. On that note, while all the NPCs in Oblivion do have audio dialogue, the game has roughly 6 voice actors and Patrick Stewart, who only does the Emperor. You will very quickly get a 'Haven't I heard you before' feeling in Oblivion.
The NWN titles aren't without their flaws, especially NWN2, however their far superior game play and modding abilities put them over Oblivion, IMO. NWN1 has thousands of mods to play, virtually unlimited replay potential. NWN2 isn't at that point yet, but there are a number of mods available now and the patches have refined the game considerably.