How long was the reign of Omri – 12 years or 6 years?

The reign of Omri was actually 6 years, but he struggled for power an additional 6 years before he achieved the rulership. After Omri became king, he ruled 6 years.

Note:

I Kings 15:25 – "And Nadab the son of Jeroboam began to reign over Israel in the second year of Asa king of Judah, and reigned over Israel two years.."

I Kings 15:33 – "In the third year of Asa king of Judah began Baasha the son of Ahijah to reign over all Israel in Tirzah, twenty and four years."

I Kings 16:8 – "In the twenty and sixth year of Asa king of Judah began Elah the son of Baasha to reign over Israel in Tirzah, two years."

I Kings 16:15 – "In the twenty and seventh year of Asa king of Judah did Zimri reign seven days in Tirzah. And the people were encamped against Gibbethon, which belonged to the Philistines."

These scriptures all reflect the reign of the kings of Israel compared to the reign of Asa King of Judah.

Now see: I Kings 16:29 – "And in the thirty and eighth year of Asa king of Judah began Ahab the son of Omri to reign over Israel: and Ahab the son of Omri reigned over Israel in Samaria twenty and two years."

The 38^th year of Asa’s reign coincides exactly with the beginning of the reign of Ahab.

Reading I Kings 16:15-23 we understand that Omri had already made himself a powerful man before Zimri killed Elah. When the opportunity arose for Omri to establish himself as king, he was in position, and used the opportunity to set himself up as king.