For WiredTiger, a background thread manages the size of the oplog, truncating it when necessary. However the piece that waits to truncate the oplog is holding onto a borrowed OperationContext. Waiting for a truncation point can be arbitrarily long, particularly when there are few writes coming into the system, or when the oplog has yet to start "rolling over".

https://github.com/mongodb/mongo/blob/24554590b43f57076035dd59629d4a4362929697/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp#L1056-L1057