The tone of the OSR is predominantly deadpan in terms of morality. Shit happens. (At times, it is simply outright dark; at others, 'weird-sad'.)
That's all well and good; but what about those times we want to try to be the good guys? (And here, I don't mean wanting to be heroes and acting out power fantasies; I mean a campaign based around trying to make a positive difference in the game world.)
This ground has been covered many times, but it bears repeating: the problem here is XP for gold.
I love the XP for gold system. It is in many ways the key to all of the doors - 'old school' play will emerge almost inevitably if it that system is used. But it has to be acknowledged that it is orthogonal to the kind of campaign I am describing. Incentives matter, and XP for gold primarily incentivises theft and looting.
XP for killing monsters is the obvious replacement, but would need tweaking to really incentivise a 'demon hunter' or 'monster hunter' campaign. Here, I picture a campaign world plagued by interlopers that are powerful but relatively rare; the PCs are a wandering band of do-gooders who try to track them down and do away with them. No moral ambiguity - these vrocks/hezrou/nalfeshnee have to go.
An alternative though, might be 'XP for rescue'. For every NPC rescued from a dangerous or horrible fate (capture, or a nearby threat, a raid, an impending disaster, etc.), the PCs win XP.
Picture the scene: the PCs foray into the abyss. In a dark chamber whose walls, floor and ceiling resemble a night sky filled with strange stars and heavenly bodies, they find three innocent people being tormented by manes. Dispatching the manes, they receive 150XP for rescuing the captives (if they can get them back to their home in the prime material plane).
Or: the PCs come to a village that is plagued by a medusa. Tracking down the monster and dispatching it and its maedar paramour, they earn 2000XP for the population of the village they have saved (400 people, multiplied by 5XP each).
The trick is simply to come up with the appropriate formula: how much XP; ought it to be based on the victims' level/HD; does one receive more for the former type of scenario (rescuing captives) or the latter (freeing a settlement from a threat)?