Well, Since Leather is my specialty, with three Leather Frags out, and more to come I can help with this, but won't give away some hard fought for bits.
Tanned Leather, like from the Tandy Store, doesn't have fragrance added.
there are some excellent bases that mimic the scent, but none duplicates it.
I've done more than 5X my share of research into the smell of real tanned leather. A 1970's era Headspace GCMS study that I found, studying just this, tanned leather from a store like Tandy's revealed the constituents. When I sent this study back to the present day GCMS specialist at this large fragrance house, and he did more research, and talked around with their perfumers, what he found is that almost all of the constituents aren't allowed to be used in making perfumes.
So then, the smell of leather must be approached form different directions, and with different methods, than trying to reconstruct it from an actual headspace GCMS.
And in all probability, the leather fragrance oils that you have access to are going to have only a general direction to leather, and won't actually be all that faithful to the odor profile that you seek.
The best ones I've found have taken me almost six years to find. One of them is listed in a book published online, but is nowhere to be found on the internet, except in that one book as a reference. It took me three years to find it after I even knew of it's existence. It is the same odor used for the Zaino "Leather in a bottle". reconditioner for leather car seats. But that version isn't cosmetic safe. There is another version that is though.
For non fragrance oils or bases, The Leather Scent profile comes through now with some quinolines, Birch tar, Cade, Castoreum and Civet, and a few other things like safraleine.
So, My recommendation to you seeking to find an FO that duplicates the scent of tanned leather, that you probably will not find it in an accessible material. And you should make do with what you CAN get your hands on, and then reshape it in the way you want to.