Myles Straw had two hits and an RBI for the Guardians, who have won four in a row overall
Cleveland starter Logan Allen blanked the A's during his four innings on the mound. He allowed two hits, struck out five and walked three.
Nick Sandlin (3-3) threw two innings of scoreless relief, followed by two more from Enyel De Los Santos. Eli Morgan pitched a perfect ninth.
Oakland starter JP Sears (1-5) went seven innings for the second straight outing. He allowed two runs and four hits, striking out eight and walking one on 106 pitches.
The A's have lost eight in a row following a seven-game winning streak.
Sandlin replaced Allen to start the fifth and walked Tony Kemp, who then went from first to third on a groundout to shortstop by Esteury Ruiz. Aledmys Diaz followed with a sacrifice fly to give Oakland a 1-0 lead.
That lead was short-lived as Bell pulled a solo home run deep down the left-field line with one out in the bottom half of the inning. It tied the score 1-1 and gave him 500 RBIs in his MLB career.
Andres Gimenez followed with a single, stole second and came home when Straw fouled off three two-strike pitches before tripling high off the wall in left for a 2-1 lead.
Gabriel Arias walked to start the eighth for Cleveland and Straw singled to put runners on the corners.
Yacksel Rios threw a wild pitch to Cam Gallagher on ball four to score a run and make it 3-1. After another single loaded the bases, Amed Rosario and Jose Ramirez had back-to-back walks to force in two more runs and stretch the lead to 5-1.
Another wild pitch by Rios made it 6-1.
--Field Level Media