Keep in mind that programs require direct observation/shadowing of an OT, which is different from volunteering. Both are certainly great to have, as it shows a diversity of experience, but know that there is a difference in terms of what you get out of each experience. When the schools have a certain number of "observation hours" required, they mean shadowing an OT as they see clients. If you call a facility and ask to "volunteer", they may stick you in an office filing papers, etc, whereas if you tell them you need observation hours for OT school applications, you will be able to watch the OTs work (generally, places do not allow any hands-on from observers, due to regulations and safety, etc). The number of observation hours required is generally stated by the schools, so you need to meet that criteria. Volunteer hours can be as much or as little as you want, though obviously a regular commitment somewhere looks a lot better than a one-and-done type of thing.
Schools may or may not require the OTs that you shadow to fill out an assessment form, asking about how professional you were, if you asked appropriate questions, how you interacted with clients, etc. Make sure you know which schools want extra forms; they should be available on the school's website with any other application materials. Volunteering is whatever else you do that is or is not directly related to the work of an OT, and does not need to be documented in any particular way except to report the number of hours you spent doing it, in OTCAS. For example, I volunteer at a respite center for kids with special needs. There are no OTs that work there, and what I do is more hands-on, so this counted for volunteer hours, NOT observation. I did my observation hours at a few different facilities where I followed the OT around, asked questions, etc. Those were noted separately in OTCAS, and additional forms were sent to schools as necessary.
