Combine oysters and buttermilk in a large shallow dish or zip-top plastic freezer bag. Cover or seal and chill at least 2 hours. Drain oysters well.
Combine flour and the next 8 ingredients. Dredge oysters in flour mixture, shaking off excess.
Pour oil to a depth of 1 inch in a Dutch oven; heat to 370°.
Fry oysters, in batches, 3 minutes or until golden. Drain on paper towels.
Stir together melted butter, hot sauce, and lemon juice. Pour butter mixture evenly over hot fried oysters. Serve oysters with Ranch dressing and celery sticks on the side.