Certain schools require PROOF you have exposure to OMM *cough cough VCOM and it's probably much easier to get that DO shadowing out of the way prior to the conditional acceptance which requires you to submit a LOR in a relatively short time frame. Unless you live in Wyoming, you SHOULD be able to at least get 4 hours of shadowing a DO. After that, if you haven't sullied your reputation by being on your phone or letting the nurse prep the room for the next patient by her/himself (sanitizing the table, making coffee, etc.), the physician would very likely let you shadow again. THEN you ask for the letter.
An alternate method involves contacting a DO school's OMM rotation site coordinator and asking to sit in on a few sessions with the instructor and third/fourth year students. Western U does this if you sign up 3-5 months in advance. Don't expect a letter, but since you have coordinated this with an office, you can provide actual contact info to that office, which looks great on your app, ESPECIALLY when applying to THAT particular school. The easiest letters to get are the private practice DOs in FM, especially when you meet the office staff IN PERSON when you ask. Have your CV, photo ID, and things like PS/MCAT info available when you go.
