Sega Master System
The original version, upon which all details in this guide are based.
Heavily based on the Sega Master System version, it changes things just slightly by removing the health meter and adding a timer. This means one hit by any enemy or bullet will kill Opa Opa immediately. Any time left over after defeating a boss is converted into money.
Converted for play on the Famicom by Sunsoft. Plays very similar, albeit slightly slower, and with fewer on-screen colors.