This is a used Blackbox Oxygen (O2) electric guitar effects pedal. This particular early production example, serial number 0214, is in very excellent all original condition and works perfectly.
The Oxygen (O2) features limiting threshold, gain, noise gate, compression ratio and “dirt” (which adds a nice analog tape type saturation to the mix) as well as compression release time/gate attack time. It also has an enabled/bypass indicator LED as it can be hard to tell if it is on or not at lower compression settings due to its transparent compression qualities. The Oxygen can be powered from a 9v battery or via the DC power jack. From light, clean compression to a crunchy, high gain sound and everything in between.