The description of opportunity attacks in the PHB (p. 195) says:
You can make an opportunity attack when a hostile creature that you can see moves out of your reach.
For a creature who has no melee attacks with only a 5 ft. reach, such as a Stone Giant whose Greatclub attack has a reach of 15 ft., would it get an opportunity attack on me if I move 5 ft. away from a position adjacent to it?
The rule states:
You can make an opportunity attack when a hostile creature that you can see moves out of your reach.
The Giant reach is unknown (see the related question), but the one and only melee attack from the stats block has 15 ft reach. If we assume its reach is 15 ft, and you are moving just 5 ft away from it, you are not "out of its reach".
D&D 5th edition empowers the DM in ways that 3rd, 3.5, and 4th did not. While rule zero has always applied, 5th edition chooses not to explicitly codify many things. Your DM might state that, regardless of the Giant's size, its unarmed attacks still has 5 ft reach, so it can kick you as an opportunity attack. This "Giant kick" isn't in the stats block though, so the final adjudication is up to the DM.
