In a traditional program, I'm guessing few people work much (maybe on weekends or something). I'm in a hybrid program that meets once a week in person, and the rest is online. Many of us work because we have to; lots of us are older, have families, aren't dependent on our parents anymore, etc. Our program technically says "no more than 10 hours a week", but the reality is that a lot of us work more than that, and it's somewhat overlooked by the faculty. The only condition is that if it were to start to affect our grades, then we'd be urged to cut back or face consequences. For me, I work more than that 10 hours a week but I also know where my limit is (somewhere between 15-20 hours a week), and so I have to say "no" to extra work, etc. In my second year especially, I've had to be really firm with my weekday boss, who was kind of jerking me around last year and trying to add more and more hours, which just wasn't possible. As it is, she knows that fieldwork, for example, takes priority so whatever day I had to go, that was the #1 thing. I've been lucky to have that flexibility, but I was up front about all of it from the start. I will obviously have to quit it in May when I start fieldwork. I also work on weekend mornings but that job is actually kind of aligned to OT and I will need it during fieldwork!
So all in all, it depends. I worked during my first graduate degree as well, which was a traditional in-class program, but it was less credits each semester. For a traditional OT program, it's probably a lot more difficult.