I struggled for awhile to identify the way to find() using a ObjectID
This seems to work, I hope this helps someone else out.
$mongoId = '5a2493c33c95a1281836eb6a';
$collection->find(['_id'=> new MongoDB\BSON\ObjectId("$mongoId")]);
I found it here: https://docs.mongodb.com/php-library/current/reference/method/MongoDBCollection-findOne/
Note this is for the PHP library, not the legacy library.