on at someone—(coll.) nagging smb. persistently; trying to persuade a person to do smth.:
- His wife’s forever on at him to do something about the fence.
on to someone—
1. = on at someone:
- She’s been on to me to buy her a new coat for a year.
2. aware of smb.’s true nature:
- You want people to think you’re just some sportswriter, but I am on to you now.
3. (Police) in hot pursuit of a wrongdoer:
- The police are on to the men responsible for that break-in.