If not breathing, give artificial respiration. If breathing is difficult, give oxygen. Take proper precautions to ensure your own health and safety before attempting to rescue and providing first aid. Adverse health effects due to ingestion to be expected, indeed they could be lethal. Call a physician in all cases, at once, and seek other medical attention.
Osmium tetroxide is noncombustible, use extinguishing media appropriate for the surrounding fire.
Special fire fighting procedures would include self-contained breathing apparatus and protective clothing to prevent contact with skin and eyes. One should use caution in the event of a fire should the sealed ampoules overhead and then explode. To prevent that from happening, continue to spray the ampoules with cold water. Should ampoules break under those conditions, the escaping vapors could come in contact with other materials and cause secondary fires. Evacuate area at once.