I answered this question a couple of years ago, and you can read that response here.

The short version of the answer is that we don't know for certain, and it would have depended on where the oath was taken. We don't have that information (three different sources offer three different locations), and in fact it is perfectly possible to argue that the oath-taking never actually occurred, and that the story was a piece of post-Conquest Norman propaganda (I go through the pros and cons of that argument in the same thread).

But if there was an oath, and if it was taken as the Brevis Relatio of c.1130 insists, then the answer is likely that it was sworn on the bones of St Pancras, whereas if it was taken in Bayeux, which is what the Bayeux Tapestry implies, it was probably sworn on the bones of Saints Rasyphus and Ravennus.