The oplogging level option in the /etc/mongodb.conf file is incorrectly named as 'oplog':

1. mongod --config /etc/mongodb.conf | head -1
   ERROR: unknown option oplog

It should be named 'diaglog' as indicated by usage

bob ~ # mongod --help
Allowed options:
...
--diaglog arg 0=off 1=W 2=R 3=both 7=W+some reads
...