After the California Angels score three runs in the top of the 1st inning, the Oakland Athletics respond with 13 of their own in the bottom half to mark the highest-scoring 1st inning by two teams in major league history. The 13 runs is one short of the record. DH Matt Stairs ties a major league mark with six RBIs in the frame. The Athletics go on to post a 16 – 8 victory.

On July 5, 1996 After the California Angels score three runs in the top of the 1st inning, the Oakland Athletics respond with 13 of their own in the bottom half to mark the highest-scoring 1st inning by two teams in major league history. The 13 runs is one short of the record. DH Matt Stairs ties a major league mark with six RBIs in the frame. The Athletics go on to post a 16 – 8 victory.

Source
Baseball Reference July 5