What is CS:CO?
CS:CO, also known as Counter-Strike: Classic Offensive, is a mod for CS:GO which recreates CS 1.6 in the CS:GO engine, the development of the current version is halted in favor for a Steam release. The community however does create hot fixes when a CS:GO update breaks something.
Their FAQ and the download can be found on Reddit.
An Steam account with CS:GO is required in order to play.