Thaw unthaw (North American usage)

awesome awful 

lesson lessen 

hi rise flats (England )

I never use the word "unthaw" it is just confusing.  I simply say thaw.

I never use the work "flat." It is a British word.

I don't get the other two.

An oxymoron is a figure of speech in which two seemingly opposing and contradictory elements are juxtaposed.  Some of my favorites are: Jumbo shrimp, awfully good, definite possibility, growing smaller, least favorite.  

@Cynique except for him rise flats, the other pairings aren't oxymorons.

But the ones i listed, that fit the definition are.   

you are right