Cut top 1/2 inch off onion; peel, keeping root intact to hold together.
Place cut-side down; make 12-16 evenly spaced vertical cuts stopping 1/2 inch from root.
Cut between sections for more petals without cutting through root.
Gently separate petals; soak entire onion in buttermilk 15 minutes.
Combine flour, paprika, garlic powder, onion powder, cayenne, oregano, salt, and pepper in a large bowl.
Remove onion; let excess drip off. Dredge thoroughly in seasoned flour, coating between all petals.
Dip briefly in buttermilk again; coat again in flour.
Preheat air fryer to 350°F; generously spray basket with cooking spray.
Place onion root-side up; generously spray entire onion.
Cook 20-25 minutes until golden brown and crispy, spraying again halfway.
Carefully remove; rest 5 minutes before serving with preferred dipping sauce.