Is omnicef (cefdinir) good for tooth abcesses?

Yes: You can use it but i would prefer to start with Amoxicillin first.

Not "good," may work: Tooth abscess antibiotic protocol for adults: pain/infection localized at tooth + use penicillin - allergic to penicillin, use clindamycin pain/infection has spread (pain around ear or below jaw line, or face has swelled) + use clindamycin + or, use a amoxicillin/metronidazole combo.