Onnia may refer to:

• Onnia (fungus), a genus of fungi in the family Hymenochaetaceae
• Onnia (trilobite), a genus of trilobite in the family Trinucleidae from the late Ordovician of Africa, Europe, North America and South America

• Mass No. 11 (Haydn)
• Mass No. 12 (Haydn)
• Mass No. 14 (Haydn)
• Mass No. 1 (disambiguation)
• Mass No. 2 (disambiguation)
• Mass No. 2 (Haydn)
• Mass No. 3 (disambiguation)
• Mass No. 3 (Haydn)
• Mass No. 4
• Mass No. 4 (disambiguation)
• Mass No. 5
• Mass No. 5 (disambiguation)
• Mass No. 6
• Mass No. 6 (disambiguation)
• Mass No. 6 (Haydn)
• Mass No. 7 (Haydn)
• Mass No. 8 (Haydn)
• Mass No. 9 (Haydn)
• Mass Notification Providers
• Mass observation
• Mass-Observation Archive
• Masso (disambiguation)
• Mass of the neutrino
• Massolin, Richard
• Massombagi