I have the GotY version of Oblivion and I've finished it once with a Fighter-type and almost finished it once as a Mage-type (though the combat grind became a little hard in the Oblivion gates). Now I'm considering restarting the game as an Argonian Thief/Assassin and play through the Thieves' Guild and Dark Brotherhood quest-lines, which I have never played as to date.

But how difficult would it be to go through the Oblivion and Shivering Isles Main Quests as a Thief (with Marksman, Blade and Light Armour, among other skills)? My main concern are the Oblivion gates, which are full of combat grind, and were tough even with a full-blown Fighter.

Tips and suggestions would be welcome!

Only things I can recommend is stay in the shadows, make good use of the 3x damage multiplier, just take it easy when your in the planes of oblivion.
Maybe make alchemy a major so you can create some nifty potions.
It all depends on your playstyle really

Going into the gates as a pure Fighter means that you're looking for trouble, because you can handle it. But if toe-to-toe fights aren't your forte, avoid them!

A thief would sneak and snipe more, and maybe use illusion magic to turn enemies against each other, or conjuration to summon allies. Use poisons to have your enemies die before they can reach you. With a ranged attack, you're much more likely to be able to tackle them one at a time, too.

I've played all the main questlines trough with the default assassin class. ...Come to think of it, that's the only character i've played all the main questline trough with.

It's been quite a while since i did it, but some of the later fights got a bit difficult (going Oblivion -> KotN -> SI), but nothing that required cheating or drastic changes in playstyle. Tough daedra at the gates were a huge PIA, but i got trough them somehow, don't really remember details, but long range sneak attacks with bows and poisons were involved. The beauty of playing a sneak character is that you get to decide what enemies you figth, when you fight them, and at what terms If you can't beat them, sneak past them, there's no reason to kill everything since Oblivion doesn't use XP-based leveling system (unless you use the OblivionXP mod ).

However if memory serves, i had my game modded with Fran's, MMM, and nGCD, so that would make big differences compared to unmodded game.

What I recommend you do, is sneak against a wall in an inn where noone can see you, use the elastic tip and do it until you're a master, then being a thief is 100 times easier than when you have to level it up.
Also, Do what the others say, sneak up behind with a dagger or a sword, or shoot them from far away, I prefer using the bow Hatreds Bow, as it does good damage for a bow.

Thanks for the info!

Another Thief question #1: would a dagger be better than a sword for this character, as both use the same Blade skill in Oblivion (unlike Morrowind)?
Another Thief question #2: would an Argonian be a better Thief/Assassin than a Bosmer? Argonians get +5 to Blade and +10 to Security, while Bosmer get +10 to Stealth and Marksman IIRC.

I'm a thief and have built up my sneak skills very nicely. I did the theives guild quests and was rewarded with a very helpful iterm for sneaking purposes. (I don't know how to put the SPOILER link in ot I would have told you what the reward is. I also picked up (and i can't remember where) the Ring of Khajiit which gives you 25% chameleon, I think it is. As a result I've been able to slip past dogs, skeletons, ghosts, you name it that are within a foot of me with complete safety.

I have done a half dozen quests in the MQ series, including shutting down the gate at Kvatch. So it can be done. If you have a decent weapon or two -- I rely on the Bow of Fire (and my marksmanship is pretty good), and you have the patience to sneak a lot (which you need as a thief anyway), you can pull it off. Have fun.

I love being a thief. I haven't done the dark brotherhood quests. Mind you,I've never visited the Arena nor gotten into theFighters Guild, either. So much to explore.

A-1 : Daggers are more suited for characters that have a low blade skill. They don't have a high base damage to start with, and rely on speed and enchantment stacking, as well as applying poisons. A character with a high blade skill is better off with a sword. Although, a dagger with the right enchantment will take out an average foe in 2 seconds or less.

A-2 : Either would make a good one. Dunmers are also a solid choice. They are prob the best, well rounded race.

Bring your sneak skill up and snipe from a distance. There is a radius/ distance at which all enemies will give a momentary pause trying to figure out where you are. That is great opportune to get in as many sneak attacks with bow as possible, provided your sneak skill is high enough (50+). Makes battles a breeze. Alternate that long distance sneak attack between foes to keep them at bay, if there are a bunch of them.
1) For melee, no. For sneak attack, maybe. If your acrobatics is 50+, then that means your sneak attacks are slower. So, in that case, a dagger is best since it swings the fastest.
2) For assassinations at the start of the game, I'd say Bosmer are better, unless you need to flee in the water and aren't too good with the lock picking mini game. Although, in the end game (maxed out stats), race doesn't matter.