Engine-O is the very first and title-bearing Engine Gattai (Engine Combination) of Engine Sentai Go-onger (2008) — the "King of the Engines" and the heart of the entire Go-on arsenal. It forms when the three core Engines unite: Speedor (a race car / cheetah, Go-on Red), Bus-on (a bus / gorilla, Go-on Blue) and BearRV (an off-roader / bear, Go-on Yellow). Unlike conventional mecha, the Engines are sentient, talking beings from the parallel Machine World who fight alongside the Go-ongers as true partners. In Power Rangers RPM (2009) the same robot is known as the High Octane Megazord, assembled there from the Eagle Racer, Lion Hauler and Bear Crawler.

Engine-O wields the Go-On Sword, stowed folded away inside Bus-on's compartment (Engine-O's left shin), together with the V Shield formed from BearRV's undercarriage. Its signature finisher, the Engine-O: Go-On Grand Prix, sends it charging at the enemy at Mach speed to deliver a single decisive slash as it races past — a checkered flag waving in the background. Like every Engine Gattai, it can also swim with surprising speed.

The truly clever part of the design is its modularity: the arms and legs are interchangeable, letting additional Engines dock onto Engine-O. This gives rise to the expanded combinations Engine-O G6, G9 and ultimately Engine-O G12, in which all twelve Engines merge into a single, colossal robot. Bandai Japan's DX toy (DX Engine Gattai Engine-O, 2008) is built around exactly this: three chunky Engine figures combine into the robot, and the centerpiece is the Engine Souls — tiny "soul" figures you slot into each Engine that trigger light and sound effects via LR44 button cells, plus firing action.

In the West it was released as the High Octane Megazord by Bandai America. For collectors, the holy grail is the full Engine-O G12 line-up: all twelve Engines including the Kyoretsu-O trio, standing roughly 28 cm tall in its final form and able to launch smaller Engines from its Carrygator feet. As the starting point of the entire Go-on building-block system, Engine-O is the foundation of any Go-onger collection.