Edit:
@MongoDB\ReferenceOne(targetDocument="Bundle1:Other", simple=true)
The option "simple" true, should work for you! The default reference format for Doctrine is DBRef()
You can query it by :
$qb->field('other')->references($OtherObject)
or
$qb->field('other')->equals(new \MongoId('01243efe..'))
ObjectId() is the native field type for mongo in PHP its \MongoId
if you want use @MongoDB\Id you have to import :
use Doctrine\ODM\MongoDB\Mapping\Annotations as MongoDB;
Take care with cache when editing annotations !
Tips : If you make getId(), this method will return a string and not a \MongoId