Clarion started out as a procedural language, but early in its Windows evolution it grew object-oriented capabilities.
When the ABC library was introduced in 1997, Clarion object-oriented programming (or OOP) went mainstream. But because of the templates and the AppGen, it's easy to work for years with Clarion without doing much if any first-hand OOP.
Part of our mission here at DevRoadmaps is to get more of our Clarion code into the hands of Clarion developers, and to do that via the DevRoadmaps Clarion Library (DCL). The DCL is a class library, and while you don't need to be an expert in OOP to use it, the more you know about object-orientation the more use you'll get out of the DCL. If you're brand new to OOP, check out Object-oriented programming in Clarion - the basics.