I agree with Hillary, as long as you seem genuinly interested in OT, then you should be fine. I had never met the OT I eventually asked to write me a recommendation. But after observing her for about 40 hours, she got to know me better than I thought she would have in such a short period of time and wrote me a really good letter of recommendation.
Also, I feel when the OT allows you to observe them, they know that being asked to write a letter of recommendation is a possibility. Make sure you give him/her plenty of time to write it (ask early/way in advance of the deadline). I asked my OT on my 3rd to last observation day (I observed with her 2-3 days a week for about a month) so that way the next session I was able to bring in all of the supplemental recommendation forms and she still had time to ask me questions if needed.
Hope that helps! Good luck 🙂