When you DO use a comma before "or":
* To separate items in a list:
* "We can go to the park, the beach, or the museum."
* To separate two independent clauses (complete sentences):
* "The sun was shining, or maybe it was just reflecting off the snow."
When you DON'T use a comma before "or":
* When "or" connects two words or phrases that are not complete sentences:
* "Do you want tea or coffee?"
* "She is a doctor, or maybe a nurse."
Example:
* "The cat sat on the windowsill, or perhaps it was just napping in the sun." (Comma used because "The cat sat on the windowsill" and "it was just napping in the sun" are both independent clauses.)
In short, you use a comma before "or" when it separates two complete thoughts, and you don't use a comma when it joins two parts of a single thought.