I think they are probably for different purposes. The oil is probably more of moisturizer and should be used early on, possibly replacing CPR or other moisturizers. The solid is used near the end of rehabbing to give it a nice sheen and protect the leather. I might try the oil someday when I run out of Leather Honey.
ETA: On Amazon, the reviews for the oil say that it darkens a lot so I probably wouldn't want to use it on a light colored leather.
So if I were to use the solid Obenauf's, do you still think using Blackrock at the end would be necessary?
The bag I'm currently rehabbing is black and it does have some color loss, so the oil darkening it would actually be a good thing. However, I wonder if the solid also darkens?