In an apparatus for controlling operation of an outboard motor having an engine to power a propeller, a transmission being selectively changeable in gear position to establish speeds including at least a first speed and a second speed and transmitting power of the engine to the propeller with a gear ratio determined by established speed, and a trim angle regulation mechanism regulating a trim angle relative to the boat through trim-up/down operation, the operation of the transmission is controlled to change the gear position from the second speed to the first speed when the second speed is selected in the transmission and the change amount of the throttle opening is equal to or greater than a predetermined value (S 10, S120, S126), and the trim angle regulation mechanism 24 is operated to start the trim-up operation based on the engine speedNE (S12, S206, S208). With this, the transmission can be appropriately controlled to suppress the decrease in boat speed caused by change of gear position, thereby mitigating an unnatural feel given to the operator.