Zonie said:
A lot of Neatsfoot oil will soften it.
With all due respect putting A LOT of any type of oil is the best way to begin it's ruin - that comment is based on 50 years as leather crafter, including being a full time maker/repairman. Leather should be in the 18-24% oil level - anything over that tends to weaken and break down the cell walls.
Any of the good saddle or leather contitioners will work but the oil should be applied sparingly in several coats if need be. Also when adding the heavier leather like neatsfoot it's best to warm it (about 110-120°F max) is good - about like hot tap water before applying and then let it "rest" for 24 hours to even out the absorbtion.
Once the oil is applied work the piece back and forth in your hands.
FWIW - neats foot oil is good (albeit most is now processed hog lard and not the old style made from calf legs), but being one of the heavier saddle/harness oils it is easy to over apply and is the number one cause for problems with too much oil in leather goods.
Other options are Lexol Conditioner (one of the best since it has been purpose designed to prevent evaporation of the oil from the leather), mink oil, Montana Pitch Blend, and one of my favs, dubbin, are some of the best available or that can be made.