The replica set will need an oplog user to access the database.
Deploying an Application with Separated Database (MONGO_URL)
You'll need to separate out your application layer from your database layer, and that means specifying the MONGO_URL. Which means running your app through the bundle command, uncompressing it, setting environment variables, and then launching the project as a node app. Here's how...
Oplog Upstart Script
Your upstart script will need to be modified to use multiple IP addresses of the replica set.
Replica Set Configuration
Then go into the mongo shell and initiate the replica set, like so: