O'Sullivan, a right-handed pitcher, will report to Class A Clearwater.
O'Sullivan, 21, was the Dodgers' fourth-round draft pick in 2011. He was 8-6 with a 3.05 ERA in 29 games between Class A Great Lakes and Rancho Cucamonga this season.
Blanton is 0-1 with a 4.91 ERA for the Dodgers heading into his Thursday afternoon start in Pittsburgh. The Dodgers acquired him on Aug. 4 after Blanton cleared waivers.
Information from ESPNLosAngeles.com's Mark Saxon was used in this report.