The fact that Leahy was not prosecuted is what leads me to think there is no actual oath - that (like so many other things!) it seems so reasonable that we just assume it's a fact.
I see a later post also points out my concern - that it could be a Republican. I know it's Machiavellian, but this is politics (grin). The reasoning would be that the WH and Conservatives would assume it was a Democrat, thereby removing suspicion from Pubbies.
Also, I find it very odd that ONE leaker would give a scoop to both CNN and FOX.
My understanding is that an oath was required by the Intel Committees after Leaky Leahy but I am still researching to find out. I will call an aide tomorrow and ask if I cannot find out.