Boss OS-2
The Boss OS-2 is an overdrive/distortion pedal, produced by the Boss Corporation, a pedal manufacturer and division of Roland Corporation. The Boss OS-2 includes both an overdrive and a distortion signal and allows for a blend of the two. The pedal was introduced in 1990 and is still being produced today.
The OS-2 combines Boss's OD-3 (overdrive) and DS-1 (distortion) circuits into one stompbox with a "Color" control knob, allowing for the user to setup a custom blend between the two signals. The pedal also includes a tone control knob which controls the tone for both signals at once.
Wolstenholme used the Boss OS-2 pedal during the Showbiz era. It provided the album's staple overdriven bass tone, together with an occasional appearance of the EHX Green Russian Big Muff Pi. It's heard on most songs off of the album aswell as most live performances from the era. Chris likely kept the color knob almost all the way to the left for a mostly overdriven midrangy sound. As the pedal can get pretty dark, the tone was likely kept open.