Does anybody know whether the NEC over-rules OSHA Standards, or is it vice-versa. Inparticular I am looking at oil filled transformers in underground spaces. [NEC 450.23 and OSHA 1926.800(s)(3)]

I am not sure but I would defer to which ever is the safest most conservative regulation. Depending on states/local jurisdiction, the latest NEC made not be the version adopted. With OSHA, since it is a federal standard there is a specific date, however, if you are located in a state with a approved OSHA program, there may be some differences between the federal and state OSHA regulations.

You must remember that the NEC is a document on how to install a safe electrical system. The key word is "How"

OSHA tells you what you must do. Key words are "what you must do" For instance, you must build a safe electrical system. You must educate employees of hazards. You must provide PPE...etc. OSHA tells you what you must do, not how to do it. The NEC tells you how to make a safe electrical system.

I agree that the NEC is a 'safe how to' document. However, the NEC also tells you what you must do in order to have a safe installation.

Both OSHA and the NEC have their own opinions as to what makes for a safe underground installation for oil filled transformers. And their opinions are somewhat in stark contrast to each other. I am leaning towards OSHA having the trump card since they are a federal organization while the NEC is a state approved document.

Any additional thoughts would be greatly appreciated.

I'm not sure I see a conflict...

OSHA 1926.800(s)(3)
Oil-filled transformers shall not be used underground unless they are located in a fire-resistant enclosure suitably vented to the outside and surrounded by a dike to retain the contents of the transformers in the event of rupture.

NEC 450.41 Location
Vaults shall be located where they can be ventilated to the outside air without using flues or ducts wherever such an arrangement is practicable.
NEC 450.42 Walls, Roofs and Floors
The walls and roofs of vaults shall be constructed of materials that have an adequate structural strength for the conditions with a minimum fire resistance of 3 hours...
NEC 450.43(B) Sills
A door sill or curb that is of sufficient height to confine the oil from the largest transformer within the vault shall be provided, and in no case shall the height be less than 4".

NEC 450.23 allows for different installation methods, if the oil has a flash point of not less than 300*C. For indoor installations, 450.23(A), you do not need a vault, or ventilation to the outside. Just a dike/containment system. Thus the conflict between the NEC and OSHA codes. The vault would not create too much of an issue with our situation, but the ventilation would since the transformers are located approximately 5000 feet underground.