Right, I'll try and keep this short. I've picked Oblivion up again. In the past I've played a melee character, but this time I'm opting to go down the stealth route. My plan is to work my way through both the Thieves' Guild and the Dark Brotherhood quest lines. I just want seasoned Oblivion vets to clear up a concern I have. 
 
My question is, would embarking on the Dark Brotherhood quest line have a negative impact on my relationship with the Thieves' Guild? If so, how? The doyen's assertion that they're "not the Brotherhood" has made me kind of apprehensive about attempting both quest lines in tandem. Any and all spoiler-free advice would be greatly appreciated. Thanks in advance, guys.

It's been a while, but I can say with a good degree of certainty that I finished the Dark Brotherhood, and then went on to do the Thieve's guild directly afterwards. I don't see any reason why, in your experience, it should be any different. 
 
In fact, I'm pretty sure that, at one point, I was at different points in all of the guild quest-lines at the same time.

My memories of Oblivion are that nothing effects your ability to do anything else, unlike the Fallout games.

Pretty sure I ran through them all

I haven't played Oblivion for a while, but I do know that I finished almost every guild quest possible on my first run (Fighter's Guild mission glitched on me, and I was never able to finish it). I think I did Thieves Guild first, and then Dark Brotherhood afterwards. I don't think any of them affect the outcome of any others, unlike Morrowind or Fallout.

The factions aren't especially complex in Oblivion, so no.

" My memories of Oblivion are that nothing effects your ability to do anything else, unlike the Fallout games. "

You should be fine. None of the guild quests for the Thieves or Dark Brotherhood have any serious impact on your standing with either guild.

" My memories of Oblivion are that nothing effects your ability to do anything else, unlike the Fallout games. "

" @SlashseveN303 said:

" My memories of Oblivion are that nothing effects your ability to do anything else, unlike the Fallout games. "

Not entirely true. In one guild quest, you have to kill a mages guild member, so anyone in the mages guild is gonna get kicked out. I think it is the thieves guild. "

All of the guilds are mutually compatible - you can finish any of their quests and quest lines in any order without little to no negative effect on your standing. However, Dark Brotherhood and Thieves' Guild quests to give you infamy, which can affect NPCs dispositions, among other things.

" @Demyx said:

" @SlashseveN303 said:

" My memories of Oblivion are that nothing effects your ability to do anything else, unlike the Fallout games. "

Not entirely true. In one guild quest, you have to kill a mages guild member, so anyone in the mages guild is gonna get kicked out. I think it is the thieves guild. "

Uh, are you sure about that? Assassination doesn't sound like Thieves' Guild's style. "

I'm pretty sure you'll be fine. I'm almost certain that for one of my characters I completed both of those guilds with no problem.

To clear up the 'last I checked' and 'pretty sure's, the definitive answer is yes, you can do them in tandem. Almost nothing you can do in that game, quest wise, effects anything else you can do in that game, quest wise. Period.

I finished every major factions' questline and was the leader of them all before it was said and done. No problemo.

I completed all 4 in one save, so yes, definitely. you'll be fine.

I'm playing this again at the moment, and currently I am doing the Fighters Guild, Thieves, Mages, Dark Brotherhood, plus I am Arena Grand Champion, among other things. So all good.
 
One thing I have noticed is the load times have decided to get marginally longer...

Being in one of the guilds doesn't have any impact on your standing in another.

" @august said:

" @Demyx said:

" @SlashseveN303 said:

" My memories of Oblivion are that nothing effects your ability to do anything else, unlike the Fallout games. "

Not entirely true. In one guild quest, you have to kill a mages guild member, so anyone in the mages guild is gonna get kicked out. I think it is the thieves guild. "

Uh, are you sure about that? Assassination doesn't sound like Thieves' Guild's style. "

Yeah. I did everything in that game there was to be done, and don't remember anything like that. "

Thanks for the responses, everyone. Glad to have that cleared up. I'm looking forward to experiencing both quest lines even more now :D

" @SlashseveN303 said:

" @august said:

" @Demyx said:

" @SlashseveN303 said:

" My memories of Oblivion are that nothing effects your ability to do anything else, unlike the Fallout games. "

Not entirely true. In one guild quest, you have to kill a mages guild member, so anyone in the mages guild is gonna get kicked out. I think it is the thieves guild. "

Uh, are you sure about that? Assassination doesn't sound like Thieves' Guild's style. "

Yeah. I did everything in that game there was to be done, and don't remember anything like that. "

my fault, you weren't supposed to kill him. "

I thought the the Thieves Guild and the Fighter/Mages guild had a strict "do not murder" policy. I always had assumed that if I started killing NPCs they would kick me out of those guilds? Is this not true?