Sources are these:
1) dump/oplog.bson (from mongodump --oplog)
2) dump/local/oplog.rs.bson (from mongodump -d local -c oplog.rs) (replica-set replication)
3) dump/local/oplog.$main.bson (same as above) (master-slave replication)
4) mongorestore --oplogReplay --oplogLimit <ts> --oplogFile <file.bson> (explicitly specify a file)

If 1 and 4 both exist there should be an error. If either 1 or 4 exist but not both, they should take priority. If neither 1 or 4 exist, but 2 and 3 both exist there should be an error.