'Post-Offer Medical' and 'Pre-placement medical' are different terms for the same service - namely an employee health exam that is made as a condition of employment, after an offer is made but before the employee is 'placed' in his position.
The term pre-placement medical is used more commonly in Canada, but many clinics do use the term post-offer in an attempt to draw a distinction between this type of medical and a pre-employment medical, which is generally illegal in Canada.
For more details see our FAQ on Pre-Employment versus Preplacement medicals:
Pre-Employment Medical vs. Pre-Placement Medical: What is the Difference?