The "one" thing is actually a whole subset that I didn't want to get into lest my post end up about five pages long! Like I mentioned, there's all these weird subsets of rules, because it wouldn't be English if there weren't about twenty exceptions for every "rule."

The one pronouns are called indefinite pronouns (they don't refer to a specific person or thing), so in that case, you're supposed to use the apostrophe s, as with other common nouns. So you'd get someone's, everyone's, no one's, somthing's, and so on. I hope that helps! To me, it just adds to my overall confusion, and I swear that if I didn't do this for work every day? I'd never know what the hell was right or wrong!