Paradox:
This is a paradox because the connections of the cube are not proportionate. If you look closely, the block that should be connecting the upper side is going behind the block that connects two parts of the cube that should be behind it. This contradicts common sense and logic because you can only draw it that way and not make it that way. It also produces inconsistency because the cube couldn't be actually made like that in real life.
You would not be able to read time, and it is inconsistent because it goes against common sense. Apart from that, if you look closely, the twelve, even though it is in a spiral, tilts a little bit to the right each time there is one complete spiral.
Oxymoron:
This signs are an oxymoron because they are right beside each other and they contradict each other since "STOP" and "KEEP GOING" are literally antonyms.
This ash tray is an oxymoron too because there is a sign of "DO NOT SMOKE" inside the ash tray; which is used when you smoke. Again, contradictory terms together. Now these are inside one another.