The Boss OS-2 is an extremely versatile pedal, great for experimenting with different sounds. However, if you're just looking for a quality overdriven tube sound, this might not be your pedal - with such a wide range of sounds it doesn't necessarily excel in any one of them.
The selling-point of this pedal is probably the "color" knob, which goes from overdrive (all the way to the left) to distortion (all the way right). In combination with its "tone" knob, you can essentially dial in anything from a warm, round, bright overdrive to a thinner, darker, almost fuzz-like distortion. The "drive" knob will allow you to manipulate the amount of gain in both cases.
The Boss OS-2 was the first overdrive/distortion pedal I owned, and after 6 or 7 years I still find uses for it and will likely never get rid of it. I have even used it as a clean boost/compressor for humbucker pickups into clean amps.
That said, in none of its settings does the pedal have the dynamics and the well-roundedness of classic tube-like overdrives. Notably, it seems to be a bit lacking in bottom-end and can sound a bit screechy unless played into very clean amps with lots of headroom. Though again, there are youtube tutorials for fairly simple modifications you can make that seem to solve this problem.
As usual with boss pedals, its housing is sturdy and well built.
All in all, a great pedal I would recommend to almost anyone.