The army made Omri king of Israel even while his predecessor Zimri occupied the throne. He promptly laid seige to Tirzah, the capital city, whereupon Zimri set fire to the palace and perished amid its ruins ending his seven day reign.
There was four years of opposition from another claimant to the throne - Tibni - but by the end of it all his rivals were defeated, and he became king of Israel. Secular historians rate Omri as one of Israel's most powerful and capable political rulers but he gets scant mention in the Bible which dismisses him as sinning more than those who went before him.
But Omri did evil in the eyes of the LORD and sinned more than all those before him. (1 Kings 16:25)
He moved his capital to Samaria where he died and was succeeded by his son Ahab.