O-Rings are used to seal the different fittings on a cornelius kegs to prevent carbon dioxide from escaping.
O-ring sizes for common Homebrewing applications
Silicone O-rings
Cornie lid o-ring size 417 Common supplier: McMaster-Carr part #9396K926
Cornie post o-ring size 111 Common supplier: McMaster-Carr part #9396K24
Cornie dip tube o-ring size 109 Common supplier: McMaster-Carr part #9396K74
Weldless o-ring for 1/2" bulkheads size 211 Common supplier: McMaster-Carr part #9396K32
Proflowdynamics camlock replacement o-ring size 211 or 210 (see here or here for example) Common supplier: oringsandmore.com, size 210 or 211
Buna-n O-rings
Cornie lid o-ring size 417 Common supplier: McMaster-Carr part #9452K218
Cornie post o-ring size 111 Common supplier: McMaster-Carr part #9452K23
Cornie dip tube o-ring size 109 Common supplier: McMaster-Carr part #9452K172 (round cross-section) or #4061T155 (square cross-section)
Cornelius Keg Poppet relief valve size 007
Cornelius Keg dome relief bulkhead seal size 020
Danco O-ring for Cornie Dip-Tube
I needed to get an o-ring right away and didn't have time to wait for shipping. I was able to find compatible o-rings at Home Depot. The inner diameter of the dip tub o-ring is 5/16". #83 O-Ring (1/2" OD x 5/16" ID x 3/32")