Your statement about OSHA hearing protection is correct, however your interpretation is inaccurate.Any hearing damage that may be inflicted in a self defense situation indoors is unlikely to be permanent. Most hearing loss happens due to prolonged exposure to high db noise. OSHA weights their hearing protection requirements in a db (loudness) vs. time (length of exposure) formula.
If a sound is loud enough (and gunshots qualify) then even a single exposure will require hearing protection under OSHA requirements and can cause hearing damage without protection.