Rosie O'Donnell -- I'm NOT Dating Tatum O'Neal

Rosie O'Donnell and Tatum O'Neal are NOT a couple ... so says Rosie.

A report Wednesday claimed the two had been dating for weeks, but that's just not the case.

Rosie's rep tells TMZ ... Rosie and Tatum have been friends for several years and met in Chicago when they both were working on shows for OWN.