Feel free to DNW specific characterization notes if you want, especially if they turn up a lot, but the problem with a general OOC dnw is it's utterly useless.
Depending on the fandom/character/prompts it might be worth saying you want it to stick with canon characterization (i.e. if you're requesting rape fic or a way-out AU) because occasionally people actually want ooc on those but tbh it's generally also useless then.
Going into extreme detail that this specific headcanon is the only possible interpretation of the character that's correct and anything else is awful get into wank territory though.
Re: Can you DNW OOCness?
Feel free to DNW specific characterization notes if you want, especially if they turn up a lot, but the problem with a general OOC dnw is it's utterly useless.
Depending on the fandom/character/prompts it might be worth saying you want it to stick with canon characterization (i.e. if you're requesting rape fic or a way-out AU) because occasionally people actually want ooc on those but tbh it's generally also useless then.
Going into extreme detail that this specific headcanon is the only possible interpretation of the character that's correct and anything else is awful get into wank territory though.