A rename collection, run from the shell, such as:
db.test.renameCollection("test2", true)
Will result in an oplog entry that contains:

  "o": {
    "renameCollection": "XXX",
    "to": "YYY",
    "dropTarget": true
  }

The following command will still drop the target collection, I assume because
{dropTarget: true} still evalulates to true.
db.test.renameCollection("test2", {dropTarget:true})

But the resulting oplog entry contains:

  "o": {
    "renameCollection": "XXX",
    "to": "YYY",
    "dropTarget": {
      "dropTarget": true
   }