Many thanks for the previous and I'd rather you relied more on your native speaker's 'feel' and not so much on dictionary data which, as often as not, lags behind the actual usage and is mostly prescriptive in nature, whereas I am after describing 'what is' and not what 'should be'.
Now, would it be right to assume that the expressions below share only ONE meaning in common? Just try substituting one for the other and see in which cases it may 'jar' on the 'sensitive native ear':
get onto someone
get to someone
get onto someone - 1. get in touch with a person: I'll have to get onto Sarah about the deadline.
2. scold or reprimand smb.: She's always getting onto the children for one thing or another.
3. keep asking smb. to do smth. and so bother or annoy the person: She's been getting onto me for a year to buy her a new coat.
4. make a person accountable for past deeds: He tricked people for years until the police got onto him.
get to someone - 1. = get onto someone 1: I got to her on the telephone and told her what to do.
2. be understood by a person: I've tried to help the troublesome boy, but I can't seem to get to him.
3. have an effect on the feelings of a person: His sad story really got to me, and I was moved to help him.
4. (US) worry, depress, or obsess a person: It depresses me, but I don't let it get to me.
5. (US) persuade a person to do smth. by bribing: The prisoners escaped after getting to the guards to leave the gate open.