Force User,
I'm sure you've been reading through these posts, so you've seen lots of good ways in which you might have avoided the TPK. Personally, I hate it when PCs die, so I feel your pain. In my case, I've got a group of mostly non-veteran players, and I'm trying to teach them how to handle encounters by working together. So far, only very limited success. If I set up encounters on the basis of what they should be able to handle, they'd all be dead.
In your case, it seems to me that your encounter set-up was reasonable. Challenging, but not too challenging. I like what I gleaned of the game flavor, too. Pagan barrows, clever opponents, etc. Although you haven't answered the question, I will assume for the moment that you took into account the spells used/damage from the barrows factor. After all, entangle would do very little underground, unless roots are dangling from the ceiling. I also imagine that your chief ogre could well get spells at night, and perform healing magic on its minions.
But, either way, it must surely be the asleep, unarmored, and entangled set-up that killed off these characters. As most agree, moving off a half-mile then lighting a fire was a stupid thing to do. Lighting a match is easily visible four miles away on a dark night with no significant cover/concealment. I would have tried to give the players some idea of the potential consequences of their actions when they decided to camp, as others have mentioned, because clearly the characters should have been either more knowledgeable or wiser than the players. Then, if they still insisted, (shrug), dem's da breaks.
As an address to the contingent who says, "Go easy on them! It's a game!", I certainly agree that the DM has an obligation to make sure the players understand what their PCs should reasonably know and sense. Other than that, I absolutely disagree with the sentiment. You do not want players who rely on you to "make things happen" or to save them from stupid planning. It is a game, and it is supposed to be fun. But, it is supposed to be fun on both sides of the DM screen.
I have had players who would happily sit in the tavern until the world ended, fully confident that the DM would save them and drop treasure in their lap. I have also had players who took the most horrendous, poorly-thought-out risks, again with the full confidence that those risks wouldn't matter because they were the PCs, and the PCs always prevail somehow.
If you don't allow the natural consequences to occur, you reinforce this kind of thinking. It can ruin games just as easily as a TPK, and it is a lot harder to undo.
Recently I had a PC lose her left hand as a result of committing a stupid theft, getting caught, and making a poor defense when her chance to defend herself came up. This after I had made sure that the PCs understood how brutal the justice system could be in the town they were in. Did I feel sorry to do it? Sure I did. Did that stop me? Not at all.
In conclusion, you could have had the ogres capture them, take all their stuff, and plan on eating them one by one. If that was the case, you should have let the ogres eat at least one of them unless they rescued themselves somehow. Then you could have had another party come in during the day, rescue the PCs, and claim the treasure (possibly/probably including the PCs gear) as their just reward. You could still do that, I suppose, and call the final blackness they thought was death mere unconsciousness. It might be useful for the PCs to gain some pointers from other adventurers. Also, if these guys keep horning in on their glory, it can set up a good-natured (or less good-natured) rivalry for sessions to come.
Making it a dream sequence, though, is counter-productive. Better by far that they start over.
RC