Rockman 2: ONN is an extensive ROM hack of the Mega Man 2 by Tatsu. Like many other extensive Mega Man romhacks, just about everything is modified, including the graphics, music, special weapons, and the boss AI. In addition to the usual robotic sci-fi aesthetic, it includes levels that appear to take place in more natural settings such as the sky or the jungle. As for the music, it includes arrangements of songs from other Mega Man games and games such as Robopon, Dragon Quest Monsters 2, and GeGeGe no Kitarou. For a tracklist of all the songs featured in this romhack, you can check out this link (Japanese only).
CUI here brings down Dr. Wily to his knees in record time, zipping through many walls and doors in the process.
WARNING: There are several instances of rapidly flashing lights during and after the Wily Stage boss fights, which may cause epileptic seizures for those affected.