The OST starts playing on Episode 4 as Sanosuke (Zanza) catches the waitress from her fall and starts trash-talking the drunk democrats.
I haven't been able to find it in the OSTs myself. It seems Sano has other OSTs considered as his 'theme', so it has been difficult to track down.
Here's a YouTube video of this scene: https://www.youtube.com/watch?v=pdSArCFcRTc
Although I couldn't find the original audio, this Spanish version does use the same music.