This was a lesson I left for my Substitute Teacher when I was away at a conference. It is an introductory Op Art lesson, hence I used it with my Grade 6's. It is fairly easy, anyone can teach it and it uses limited materials (only markers). I found the lesson here on the Incredible Art Deaprtment website and altered it a bit.
To be honest, I thought this lesson would be a bit boring, as it's just so much repetitive coloring. In the end, some kids did complain a bit (the rushers- heh, heh), but not as much as I expected. I often ask my students how they are enjoying the project if it's the first time I'm teaching it and they are very honest with me (sometimes too much!) -- it is a good gauge for me whether or not I will do the lesson again.
With this one, the paper was quite large, so next time I would just use regular 8x10" (A4) paper. One warning- some of the markers ran out; other kids couldn't remember which colour they used the previous class- something to keep in mind if you teach this.
Trace a variety of sizes of circle (we use plastic lids). Draw slightly wavy or straight lines in one direction; rotate the paper and draw some more lines. Don't draw too many lines or it gets too complicated to colour. Mark every other 'section' with a dot- this will be where the darker colour will go. These seem to look best when two very contrasting colors are used.