Most languages I have used feature OOP elements. In fact, I don’t think I have used any languages that don’t feature OOP outside of Linux shell scripting, which I do not have much experience with. I have not used OOP outside of school extensively but I have on occasion. I have used it recently when learning Flutter for mobile app development. I have used UML diagrams as well, I have not found them to be particularly useful at the scale of my work, but I can see how they could become useful when projects become very large.