I've been surprised that recently many people have already known what OT is. I guess it's because some older relatives have been patients of OTs (for wrist breaks, etc.) and two of my in-laws have an adopted child who needs to work with an OT at school. So, they all have at least a narrow idea of what OTs do (though the ideas are probably very different from each other.)
Until very recently, though, NO ONE has had any clue what it was. Given my previous work in higher education and the word "occupational," many have assumed it's something to do with career counseling or training.
My automatic response now is to mention occupational therapy, and then say that it's a "rehabilitative field." Then people usually nod in at least some vague understanding. Or I might say that "it's a field in the rehabilitative sciences," and, not yet being an OT myself and therefore not yet working in just one area, I say OTs might work with people with cognitive or physical deficits to improve their functioning in daily life. That's still vague, but people get the idea. If you think about it, most people, even in broad or lesser-known fields don't explain in great detail what they do. My husband might say he is a businessman, or works in "management" or "operations" or something like that, but he does not detail every aspect of his work. Business is really very broad, but people will get the general idea of what he does.