An outboard motor for a marine vessel application, transmission devices for such an outboard motor, and related methods of making, operating, and modifying attributes of same, are disclosed herein. In at least one embodiment, the motor includes a horizontal-crankshaft engine in an upper portion of the motor, positioned substantially above a trimming axis of the motor. In at least another embodiment, first, second and third transmission devices are employed to transmit rotational power from the engine to propeller(s). In at least a further embodiment, the motor is made to include a rigid interior assembly formed by the engine, multiple transmission devices, and a further structural component. In further embodiments, the motor includes numerous cooling, exhaust, and/or oil system components, and/or other transmission features. In at least some additional embodiments, a transmission device of the motor is configured to facilitate gear ratio variation and/or includes an integrated oil pump.