| field | optional | description |
|---|---|---|
| ts | N | BSON Timestamp. Often converted to 64bit INT for JSON |
| h | Y | Random hash |
| v | N | oplog protocol version. default 1. |
| op | N | Type of op. one of i, u, d, n, c |
| ns | N | BSON Namespacestring. Serialised as db.collection |
| o | N | Operation applied. object |
| o2 | Y | Object 2. Additional information, usually operand |
| fromMigrate | Y | Boolean. Indicates if this operation is part of chunk migration between shards |
source: https://github.com/mongodb/mongo/blob/master/src/mongo/db/repl/oplog_entry.idl