1)
Why does it say "Oso Hechel"?
1.
Rashi: It was the first Mizbe'ach that Sha'ul built. Radak - in Gilgal, he offered on Mizbechos that were already built.
2.
Radak citing Bamidbar Rabah 10:1: Many built Mizbechos before him. He was the first king to build a Mizbe'ach.
3.
Radak citing a Midrash: Sha'ul put the first rock, and others completed it.
4.
Malbim: Because people were rushing [to offer their Korbanos], he began with an improperly built Mizbe'ach. Later, he built it properly.
