If your lease actually has an exclusivity clause in it that prevents the landlord from leasing to an occupational therapy business, you can sue the landlord for breach of contract.
However, for this to work, such a clause must be very specific - exclusivity clauses are generally very narrow and would limit only direct competition, not just competition from the same type of business (for example: a clothing business will generally be able to secure an exclusivity clause for a specific brand, but will not be the sole purveyor of clothing in an entire complex).
It is very important to have the specific language in your contract (not just that clause, but the entire lease) reviewed by a local attorney prior to engaging in litigation (think of this as squeezing toothpaste out of the tube - once it is out, it is impossible to get back in - you want to make certain that you are on the right side of the contract before filing suit).
You can find local attorneys using the State and local Bar Association directories, or private directories such as www.AVVO.com; www.FindLaw.com; or www.Martindale.com (I personally find www.AVVO.com to be the most user friendly).
Short of filing a lawsuit, you can try to mediate the dispute with them - contact your local bar association and request referrals to mediators, a third party neutral can often help you reach a mutually agreeable resolution. Use the bar association's referrals to contact a mediator or two, the mediator will then contact the other party to set up a mediation session, and you can go from there - hopefully resulting in a formal or written settlement agreement, and save yourself the time and expense of litigation.