Oobleck
Oobleck itself may be a symbol for the destruction and manipulation of nature. Mankind has destroyed the current climate and is continuing to abuse nature and its resources. As a result, oobleck may signify the destruction of nature.
The King
The King is a reference to the greed and power-hungry nature of man. Mankind continuously is ungrateful for the things they have and always seek more. When the King demands control over the skies, he has let the power get to him and he tries to change the situation in his favor, by forcing the creation of ooobleck.
Bartholomew
Bartholomew himself is a symbol for the bravery and hope that humans can inspire. He endures relentless challenges in his pursuit to get rid of oobleck and is eventually successful in this.
Sky
The sky itself is a symbol of God’s existence. When the King apologizes to the sky, it is like he is apologizing to God. As such, the skies/God forgives him and gets rid of oobleck.