While publishing Sandbox, I have encountered below issue couple of times.This can be because of numerous reason. Multiple people updating at the same time but in my case I was the only one using OIM so I find below workaround useful when i hit this issue.
Step 1. Export the sandbox to your local directory.
Step 2. Delete the sandbox from OIM.
Step 3. Import the Sandbox in OIM, Activate it and hit the Publish button. Works for me 🙂