Should occupational therapy be capitalized in a sentence?

No, the compound noun 'occupational therapy' is a common noun, a general word for a type of rehabilitation process. A common noun is only capitalized when it is the first word in a sentence.

The common noun 'occupational therapy' functions as a proper noun when it is the name of a specific department in a medical facility or the name of a commercial enterprise, for example, The Occupational Therapy Program at Children's Hospital Colorado in Aurora, CO. A proper noun is always capitalized.