well, the first time I saw an ameboid star, and then very shortly I saw an invasive (where I live) oak's sapling. I'd say it's not an OI, it just makes you search for the focus of the image. The classic example Lumifer links to is something where you might not even think of looking for the other image; and of course, there should be ambiguity things nearer to crystal clear end of spectrum.
Would you say the picture below (by A. S. Shevchenko) is almost like an optical illusion?
Have you seen any pictures or sights that fooled your brain for a moment but that you wouldn't call optical illusions, and if yes, what is the salient difference?