"Classic Offensive is a CS:GO mod with the goal of bringing back the community aspects and aesthetics of CS."
For now, you can only play the outdated beta version that has been published on ModDB.
Since the last public release, a lot of progress has been made and a lot of new features have been added to the mod.
The definitive version of Classic Offensive is set to release on Steam sometime soon...
ModDB Beta version installation:
1) Download "csgo_legacy" from CS2 beta branches.
2) Download Classic Offensive Beta 1.2.5c and 1.2.5d Patch.
3) Put the files inside "C:\Program Files (x86)\Steam\steamapps\common\Counter-Strike Global Offensive\".
4) Add "C:\Program Files (x86)\Steam\steamapps\common\Counter-Strike Global Offensive\csgo.exe" as a Non-Steam game.
5) Add "-steam -game cscomod/csgo" to the launch options.