class MyUberSchema(OneOfSchema):
    type_schemas = {
        'foo': FooSchema,
        'bar': BarSchema,
    }

    @post_load
    def whatever(self, data):
        print("post load")

AFAIU, post_load decorated method is never called. It may be intended, or it may not be an issue, but while trying to do that it failed and I didn't see this documented so I don't know if this is known.

To get this to work, the post_load method needs to be defined in FooSchema and BarSchema (or a common parent).