In a customer case configuring the oplog to 3TB resulted in memory usage growing over time, apparently without bound.

Reducing the oplog size resolved the customer issue, but doing some testing around huge oplog configurations seems warranted.

EDIT:
Apparently the size of the oplog was not reduced, however, metrics show the application reduced the amount of oplog GB/Hr. Obviously this introduces the possibility that oplog deletion is lagging, it's not the size of the oplog that is the principle concern.