Most Op Art projects are formulaic and therefore not hard to do, but they yield a wow factor. Plus it is just fun to talk about optical illusions, even to students with optical challenges.
For the squiggle project, you draw curved lines from one end of the paper to the next. Most of mine went top to bottom for a horizontal piece of paper. Then they drew a curve from the middle of one line to the next, like a smile. When you get to the next line make it a frown, and continue going smile, frown, and switching direction at each line. After that every line above the line will be frown lines, and every line below the line will be similes. This is the same principle that is used when drawing cylinders with the horizon line (eye level) somewhere in the middle.
For my students who have no vision, I used hot glue to trace their initial lines, so they could draw their "smiles" and "frowns" within those boundaries. Then I created construction paper stencils of crescents of various widths and thicknesses. I would have to help pick out the right size stencil, but at least these students could color their piece independently.
It is not only the repetition and color contrast that creates the optical illusion, but the value. By shading the colored pencil on the edges, or the ends of each crescent shape, it gives the illusion of three dimensions. This project took a couple of weeks, but it was well worth the extra time to see the sense of accomplishment in my students' faces for a job well done.