It's possible to create an oplog (whether by --oplogSize, replSetResizeOplog, or manually) where the capped maxSize is larger than the total size of the storage volume which holds the oplog. In this case, the oplog is basically guaranteed to (eventually) fill that disk, ie. without intervention the mongod can be expected to fail at some unknown time in the future.

mongod should check the maxSize of the local.oplog.rs collection, and issue a startup warning if it exceeds the total physical size of the storage volume which that collection is stored on. This will alert users to this situation, so that it can be corrected before it becomes a problem. The check should be done after any new oplog has been created, as well as after the size has been changed with replSetResizeOplog (or perhaps check during this command, and refuse to run (or require force: true in that case).