If it sounds like "Oh Well" was recorded by some other Fleetwood Mac, that's because it basically was. Sure, drummer Mick Fleetwood and bassist John McVie have been in the group from the very beginning, but with Peter Green at the helm initially, they were a very different sort of beast. "Oh Well" is perhaps the most famous song of this era.
The blues-rock standard was written by Green in 1969, and it's easy to hear early Led Zeppelin and other 1970s hard-rock bands in the tune. Garage rock bands have been playing it for decades. It's also been covered by everyone from Tom Petty to Ratt to the Black Crowes. The group kept it in the setlist for years, and Lindsey Buckingham did a pretty great job singing Peter Green's parts.