Everyone is familiar with the term Pop Art and the works associated with it, right? Well, Op[tical] Art is a pun on "pop art". Josef Albers (that guy who paints the squares within squares) got the ball rolling with optical experiments and color theory back in the 1920s. By the time the 60s rolled around (which is when Pop Art emerged), there were a handful of artists exploring the idea of creating artwork that looked like it was constantly moving, hence the "optical" in the term Op Art. Op Art relied on illusions and mathematics to create its essence.
In the 50s, Abstract Expressionism (think Pollock, De Kooning, and Kandinsky) was at its peak, and the Op Artists wanted a way to organize that chaotic feeling and non-representational ideas that the Abstract Expressionists embodied. In other words, art didn't have to be about things that we identify with out world. Abstract Expressionists created work that wasn't found in nature and didn't mimic anything from reality. The Op Artists took their ideas a step further by incorporating math into their designs, creating an effect in their artwork that looks like the piece is moving.
Op Art was an art movement that was seen all over the world. Two of the most well-known Op Artists are Bridget Riley (from the UK) and Victor Vasarely (a Hungarian). MC Escher also worked with some Op Art designs, which is interesting to point out because a lot of his work is very mathematical.
Op Art fell out of the limelight by the end of the 60s, but a lot of these artists are still taught in foundational art courses as a means to learn control of ones material. It's been quite often that I've gone into a classroom (at just about any level; elementary or secondary) and have observed students creating their own Op Art designs to master colored pencil, markers, acrylics, or gouache. It takes a surprising amount of dexterity to be able to create a precise design, which is why Op Art is valued in the classroom today.
You can learn more about Op Art here!