By inspection, it is possible to drop local.oplog.rs while a mongod is running as a replica set member (primary or secondary):

rs0:PRIMARY> db.oplog.rs.drop()
true
rs0:PRIMARY> show collections
me
slaves
startup_log
system.indexes
system.replset

When this operation is performed on a node running as primary, the next write operation appears to cause the server to crash:

rs0:PRIMARY> use test
switched to db test
rs0:PRIMARY> db.people.insert({"name":"Jones"});
2014-06-10T16:45:40.891-0400 DBClientCursor::init call() failed
2014-06-10T16:45:40.892-0400 Error: error doing query: failed at src/mongo/shell/collection.js:258

(Clearly this behavior is not intended to be safe, however, it seems as if it would be possible to prohibit such unsafe operations.)