diff --git a/check_mongodb.py b/check_mongodb.py
index 9f96116..3484fd0 100755
--- a/check_mongodb.py
+++ b/check_mongodb.py
@@ -211,9 +211,9 @@ def main(argv):
     if action == "connections":
         return check_connections(con, warning, critical, perf_data)
     elif action == "replication_lag":
-        return check_rep_lag(con, host_to_check, port_to_check, warning, critical, False, perf_data, max_lag, user, passwd)
+        return check_rep_lag(con, host, port, warning, critical, False, perf_data, max_lag, user, passwd, authdb)
     elif action == "replication_lag_percent":
-        return check_rep_lag(con, host_to_check, port_to_check, warning, critical, True, perf_data, max_lag, user, passwd, ssl, insecure, ssl_ca_cert_file, cert_file)
+        return check_rep_lag(con, host, port, warning, critical, True, perf_data, max_lag, user, passwd, authdb, ssl, insecure, cert_file)
     elif action == "replset_state":
         return check_replset_state(con, perf_data, warning, critical)
     elif action == "memory":
@@ -402,7 +402,7 @@ def check_connections(con, warning, critical, perf_data):
         return exit_with_general_critical(e)
 
 
-def check_rep_lag(con, host, port, warning, critical, percent, perf_data, max_lag, user, passwd, ssl=None, insecure=None, ssl_ca_cert_file=None, cert_file=None):
+def check_rep_lag(con, host, port, warning, critical, percent, perf_data, max_lag, user, passwd, authdb="admin", ssl=None, insecure=None, cert_file=None):
     # Get mongo to tell us replica set member name when connecting locally
     if "127.0.0.1" == host:
         if not "me" in con.admin.command("ismaster","1").keys():
@@ -511,7 +511,7 @@ def check_rep_lag(con, host, port, warning, critical, percent, perf_data, max_la
                 lag = float(optime_lag.seconds + optime_lag.days * 24 * 3600)
 
             if percent:
-                err, con = mongo_connect(primary_node['name'].split(':')[0], int(primary_node['name'].split(':')[1]), ssl, user, passwd, None, None, insecure, ssl_ca_cert_file, cert_file)
+                err, con = mongo_connect(primary_node['name'].split(':')[0], int(primary_node['name'].split(':')[1]), ssl, user, passwd, None, authdb, insecure, cert_file)
                 if err != 0:
                     return err
                 primary_timediff = replication_get_time_diff(con)
@@ -1169,15 +1169,18 @@ def check_oplog(con, warning, critical, perf_data):
     critical = critical or 4
     try:
         db = con.local
-        ol = db.system.namespaces.find_one({"name": "local.oplog.rs"})
-        if (db.system.namespaces.find_one({"name": "local.oplog.rs"}) != None):
+
+        # check if replicaset member
+        ol = db.oplog.rs.find_one()
+        if ( ol != None):
             oplog = "oplog.rs"
         else:
-            ol = db.system.namespaces.find_one({"name": "local.oplog.$main"})
-            if (db.system.namespaces.find_one({"name": "local.oplog.$main"}) != None):
-                oplog = "oplog.$main"
+            # check if master/slave setup
+            ol = "db.oplog.$main.find_one()"
+            if ( ol != None):
+                oplog = 'oplog.$main'
             else:
-                message = "neither master/slave nor replica set replication detected"
+                message = "neither master/slave nor replica set replication detected. Could not determine oplog location"
                 return check_levels(None, warning, critical, message)
 
         try:
@@ -1187,8 +1190,8 @@ def check_oplog(con, warning, critical, perf_data):
                 data = con.admin.command(son.SON([('collstats', oplog)]))
 
         ol_size = data['size']
-        ol_storage_size = data['storageSize']
-        ol_used_storage = int(float(ol_size) / ol_storage_size * 100 + 1)
+        ol_storage_size = data['maxSize']
+        ol_used_storage = int(float(ol_size) / float(ol_storage_size) * 100 + 1)
         ol = con.local[oplog]
         firstc = ol.find().sort("$natural", pymongo.ASCENDING).limit(1)[0]['ts']
         lastc = ol.find().sort("$natural", pymongo.DESCENDING).limit(1)[0]['ts']