(Disclaimer: I'm sorry, this is sort of lame but I had to blog about it anyway. Because I need to write down stuff in order to remember it. Because I had to look this up at least 10 times now. And because I always need to debug/search for the same info (which brings me back to my first 'because'). And because I still don't understand why OpenX/Ads doesn't have a simple user table. :-))
OpenAds 2.3.x-beta, 2.4
Columns: admin, admin_pw, ...
Depending on your install, the table name might have another prefix than "oa".
The password is a simple md5 hash, which means you don't need anything funky to replace it in the database.