The Observation Post is a Tech Building first seen in the real WWIII. It is essentially a radar that, once captured, manages to permanently (at least as long as the structure was in a Commander's hands) remove a large part of the fog of war. Capturing an Observation Post near an enemy base would make a Commander always aware of what the enemy is doing. So control over these structures is often useful.
Observation Posts are only placed on land. However sometimes they're placed on very small islands, so they act almost like if they were placed on the sea.
Interestingly, its appearance is similar to that of a radar tower, and radars are not used in WWIII.