The phrase "onned the tab" can be used in a number of different contexts, from describing someone who has paid a bar bill to someone who has opened a new webpage on their computer. When looking for synonyms for this phrase, one could use "settled the account," "paid in full," "closed out," "overturned," or "activated." Depending on the specific context, other possible synonyms might include "bought a round," "covered the bill," "cleared the slate," "shut down," or "loaded up." Ultimately, the best synonym for "onned the tab" will depend on the intended meaning of the phrase and the specific context in which it is being used.