The central midfielder has joined on a free transfer from Dorking Wanderers.
The 23-year-old played a big part in helping the Surrey club win promotion from the National League South through the play-offs last season, scoring four goals in 25 appearances after joining from Hythe Town in November.
Oldaker has also played for Gillingham, scoring five times in 33 appearances.
He said: “I’m absolutely delighted to sign for such a big club. I literally just want to get going now.
“It was tough moving away and moving everything up here, but I’m more than happy to do it. I can’t wait.
“He’s clear of what he wants from me as a player at this club and I’m ready to do it for him.”
As well as Dorking and Gillingham, Oldaker has also played for Billericay Town and Hythe Town.
Dorking, who have never faced Chesterfield, won promotion to the National League after beating Ebbsfleet United in the play-off final.
Oldaker joins Branden Horton, Joe Cook and Bailey Clements in joining Chesterfield this summer.