Its not recommended to put different brands on all four wheels or on each axle, or is it against the law. However they must have the same load and speed rating. the tyre must also have the same makeup raidia or crossply and have the same tread pattern.
OEM Tyres fitted to mass market vehicles by the manufacturer are sourced by cost to to supply and what the manufacturer chooses to fit based on the car. If the OEM tyres fitted to your car are Continental, it is pefectly legal to change to Michelin or any tyre brand, as long as the load rating and peed rating match the manufacturers specs.