Hi!
So I volunteered/observed at 4 different settings and received LORs from two.
All OTs are different and some may/may not agree to write you a LOR so I would suggest running it by before you volunteer/observe for too long.
From my personal experience: First, I was at a hand therapy clinic (~80hrs) and I asked the therapist privately (after clinic hours) if she could write me a LOR and she told me she only writes LORs for her aides. I was SO upset and thought I wouldn't be able to get a LOR in time for my apps, but I landed a short-term summer agreement at a peds camp (~33hrs) and I eventually asked the therapist on my last day and she was thrilled to write one for me. I then observed at a different clinic (~40 hrs) and she eventually asked me if I wanted a LOR (which I said yes! to). I was also at a hospital outpatient facility (~30 hrs) and the therapist also asked if I needed a LOR (which I denied as I had two therapists willing to write LORs on my behalf).
OTs know the reason why you're volunteering/observing and most will approach/ask you if you need a LOR (especially if you got close to them/have a good relationship with them); however, it's smart to run it by before spending too much time at one setting. As for asking them I would suggest talking to them before/after clinic hours or whenever the therapist isn't with patients/has free time or even via email! Hope this helps!