Olt entered the game as a pinch-hitter in the eighth and stroked a 1-0 pitch over the left-field wall. Albert Almora, who smacked an RBI single earlier in the frame to break a 3-3 tie, also scored on Olt's first homer of the spring.
Justin Ruggiano clubbed a two-run shot and finished with three RBI, while Anthony Rizzo added two hits and scored twice for the Cubs, who saw starter Kyle Hendricks allow one run on one hit and three walks over two innings.
Brandon Moss and Josh Reddick each registered two hits and an RBI for the A's. Oakland starter Tommy Milone gave up three runs on five hits in three frames.