...whose frown And wrinkled lip and sneer of cold command (4-5)
We know that later in the poem Ozymandias will brag about the greatness of his works, but here he seems less than satisfied with something, as if he thinks his works could be better. We can imagine the sculptor hammering away at the statue and Ozymandias giving him a dirty look because something about it just isn't right. Alternatively, perhaps Ozymandias was perpetually frowning because his empire just wasn't good enough, or big enough.
"My name is Ozymandias, king of kings: Look on my works, ye Mighty, and despair" (10)
There is a lot of arrogance in this statement, and it's almost as if he were saying that his name means "king of kings." He brags about his "works" (statues like the one described, pyramids, etc.) as well, telling the "Mighty" to "despair" because their works will never be as good or as his. Ironically, Ozymandias's works are nowhere to be seen – all that's left is a barren desert and this broken statue. His pride is made to look stupid because his "works" are all gone, except for this fragmented statue that, quite literally, is on its last legs.