We are implementing the odm and the orm. The namespaces with the softdelete bundle in this setup create a conflict since the orm setups the doctorine name space in a different location then the softdelete bundle. Which ever is listed first will win out.
This bundle uses the Doctrine\Bundle\MongoDBSoftDeleteBundle namespace, which doesn't conflict with anything in ORM. If you examine composer.json, you'll see that autoloading should be configured for the full namespace.
I assume you're not using composer, but feel free to share your autoloader file and I can point out a solution.