My wife is a contracted Occupational Therapist for three agencies, in addition to her OT work for a school district. Is all mileage from her home office deductible?

[MinhT1]

Mileage from home and back is commuting mileage and is not deductible.

In your wife's case, if she has to go to several places of work in a day, the mileage from home to the first place of work and from the last place of work back home is not deductible. Mileage between work places is deductible as a business expenses.

[Hal_Al · Accepted solution]

Simple answer: Yes.

The key words are "contracted" and "home office". By contracted, I assume she does not get a W-2 for her pay. That makes her self employed and they are allowed to deduct qualified buiness mileage.  To deduct ALL mileage, the home just needs to be the "Principal Place of Business".  It does not need to qualify for the Home Office deduction (the 'Regularly and Exclusively' rule does not apply).  
https://www.irs.gov/pub/irs-drop/rr-99-7.pdf
https://www.irs.gov/publications/p463/ch04.html

If your home is not her Principal Place of Business, then she may not deduct mileage to and from clients, but may deduct the mileage between clients, if she goes to more than one work place in a  day.  However, if she also has a W-2 job ("OT work for a school district"), she may deduct the mileage when she goes directly  from the W-2 work site to a client.