Heat a grill to high (450° to 550°). In a small bowl, combine mayonnaise and Sriracha; chill. In a glass pie pan or baking dish, combine oil, garlic, oregano, cumin, salt, cayenne, and pepper. Turn fish in seasoned oil to coat. Seal tortillas in foil.
Grill corn, covered and turning a few times, until lightly charred, 8 to 12 minutes total. Five minutes after putting corn on grill, set a large cast-iron skillet or other ovenproof frying pan on cooking grate. After a few minutes, put fish in hot skillet and tortilla packet on grate. Cook, turning fish and tortillas once, until fish is no longer translucent in center (3 to 5 minutes) and tortillas are hot (4 to 5 minutes). Transfer to a board.
Cut kernels off corn in slabs. Spread tortillas with some of Sriracha mayo and put the rest in a small dish. Fill tortillas with chunks of fish, avocado, cabbage, cilantro, and corn. Serve more Sriracha mayo on the side.
Wine pairing: Groth 2015 Hillview Vineyard Chardonnay (Napa Valley; $30)