[klibertp]

Could you tell us what kind of experience you have? There are many ways of learning OOP and they vary greatly in how approachable they are depending on your previous experience.

If you come from a procedural programming in a low-level language like C, for example, there is an excellent book "Object Oriented C", which teaches you OO concepts by implementing them in C.

If you do GUI programming in a procedural language (again, mostly C) you could download Pharo and go through "Pharo by Example".

These are just examples - there are much more languages/books combos out there.