Place the onion slices in a large bowl and cover with the buttermilk. Allow to soak for 15 minutes.
Mix together the flour, salt, black pepper, chipotle powder, oregano and cumin. Taste and adjust seasonings. Divide the flour mixture, placing half in a large food-safe plastic bag and the other half on a plate.
Heat up 2 inches of oil in a large pot or cast-iron skillet to 350° F. Line a large baking sheet with paper towels.
To batter the onions, remove them from the buttermilk, reserving the buttermilk in the bowl, and place them in the plastic-bag with the flour. Shake until coated. Working one-at-a time or in small batches, take the floured onions and dip back into the buttermilk and then dredge in the flour on a plate. Fry in the hot oil until light brown, for about 2 minutes, turning once. Drain on paper towels.
Serve warm with ketchup or your preferred dipping sauce.