O'Shea had smacked an effort onto the crossbar early on for the hosts after good work from Ollie Banks.
Labadie looked particularly dangerous for the visitors and dragged another effort past the wrong side of the post.
Torquay eventually made the breakthrough five minutes after the restart when Shamir Goodwin slotted home to put them in front.
Armand Gnanduillet twice came close for Chesterfield, before O'Shea put them in front by firing across goal and into the bottom corner.
O'Shea then sealed the win by scoring just before the full-time whistle.