I am quite sure it was not Osage for a couple of reasons. Primarily, this is a flora of the Great Plains and first found in Texas. This fence of which I found a post of had to date back before 1888 or before RI's great fire which burned almost half the state. It was charred so it was in the fire. I found this post about 20 years ago and it had no rot once I pulled it out and examined it. Also, Osage burns like crap but black locust burns about as good as you would want a piece of wood to burn. Osage (if it grew here) would have exploded itself in this fire. New England was full of black locust. They used it for posting, fuel and even rr ties. Any cedar fencing I have put in was shot after 25 years. Black locust is a legend in these parts as the 100 year post. The only variables I can think of is that perhaps it is honey locust you are using or differentiated soil conditions and micro organisms attack certain woods more readily than others depending on the part of the country. After I found this post, I did bring it to the local library as the towns historian resided there. This gentleman was 88 years old at the time and gave me the complete history of the entire area as well as a nature lesson. This guy was hopping through my woods as if he was a 20 year old an found 2 more posts which he clearly identified as black locust. He died at a 102 but what a resource he was. I am puzzled why you are not getting the longevity purported to be inherent of black locust.