Can I mount an over the range microwave in a cabinet?
Microwaves do require ventilation - there are typically cooling fans and airflow is required for them. Many "countertop" models include specifications for required ventilation when mounted in a cabinet. 

If the "over the range" model you are considering also has a "range hood" function that might call for more ventilation, but even if not using that function some ventilation is required. I would suggest calling the manufacturer's technical support line and asking questions - I'd also suggest at least considering another model more suited to your actual use scheme, if it is one that includes a "range hood" function that would be of no use in your desired location.