You must actually dump the oplog using mongodump -d local -c oplog.rs which will create a folder called local with oplog.rs.bson in it. However, when you do a mongorestore --oplogreplay, it will look for a file called oplog.bson which is only created by doing a mongodump --oplog. So you actually need to name the oplog.rs.bson to oplog.bson and move it up a directory for this to work which is very messy. If mongorestore can't find oplog.bson it will tell you to use mongodump --oplog which is just plain wrong.