Do you mean you have the 24 hex digit string of an ObjectId?
Assuming that's what you mean, most drivers have a way to take a string and convert it to an ObjectId. In JavaScript that's:
.find({_id:new ObjectId("4f91bfcfaa7c5687a0c686d4")})
Updated to be a bit more useful for the node-native driver (from the documentation at https://github.com/christkv/node-mongodb-native):
// Get the objectID type
var ObjectID = require('mongodb').ObjectID;
var idString = '4e4e1638c85e808431000003';
collection.findOne({_id: new ObjectID(idString)}, console.log) // ok
collection.findOne({_id: idString}, console.log) // wrong! callback gets undefined
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