OOPArt

The OOPArt stands for Out of Place Artifact. It was a device used by the Lost Tribes, each was wiped out by their specific OOPArt. Dark Phantom says that when the weak use the OOPArt, they can't control it and lose themselves in the process, and will be consumed by it.

At the IFL Tower Museum, an OOPArt was on display (it varies depending on the game's version). Omega-Xis noted that the OOPArt was emanating strong EM waves. When Rogue attacks the museum, he attempts stealing the OOPArt. MegaMan fights for the OOPArt, only to end up with Omega-Xis accidentally swallowing it.

Following this, the sprits of the Tribe started talking to Geo and Omega-Xis. First they sounded insane, bugging MegaMan to give them his body. When MegaMan is confronted with several Errands at the same time, the spirits noticed his plight, and taught him how to use their power, allowing MegaMan to wield it and defeat the Errands.

The OOPArt was eventually extracted from Omega-Xis by Hollow, and was used by Dr. Vega to resurrect Mu. After Le Mu's destruction, the continent of Mu is destroyed, and the OOPArt is lost in the ocean.

In the Dimensional Gorge, after defeating Apollo Flame for the first time, the spirit of the OOPArt returns, congratulating MegaMan for averting a future of destruction. It also reminds MegaMan on the fragility of bonds, which was what resulted in the Tribe's downfall.

Trivia

 * In programming, "OOP" stands for Object Oriented Programming, a type of programming design which uses "objects" to interact with each other. The name OOPArt is most likely a reference to this.