The word or works just like the word and.
In a series when the conjunction, the word or, appears between each member of the series, use no commas.
...tan or blue or silver...
...25 or 28 or 31...
In a series when the conjunction, the word or, is at the end of the series and there is no other conjunction, use a comma before the word or (just as you would before and).
...tan, blue, or silver...
...25, 28, or 31...
Otherwise, use a comma before the word or when there is an independent subject and verb after it.
...go with her, or he will have to stay here...
...go with her or will have to stay here...
...do this, or she has no choice...
...do this or has no choice...
Happy punctuating!
Margie