Collusion attack is an attack against Optimised Link State Routing (OLSR) protocol in mobile ad hoc network. Two malicious nodes work together so that routes to the target (victim) node(s) are not established in the network. Multipoint relay (MPR) selection process in OLSR is exploited to achieve this route denial. Packet delivery ratio for the target drops to 0% for nodes at distance of 3 hops or more. In this paper, we propose use of convergence characteristics of OLSR in designing effective, resource efficient countermeasures for packet dropping attacks such as collusion attack. We propose a detection method and countermeasure Scruple where discovery packets called ScruplePackets are injected to probe routes from target node to its three-hop neighbours. Acquired information is analysed to conclude if the node is a target of collusion attack. We also propose a novel attack-resistant method named Forced MPR Switching OLSR in which a node temporarily blacklists potential attackers on observing symptoms of the attack, thereby, forcing recomputation of its MPR set. Proposed approaches incur minimal penalty on network performance. Simulations conducted on Network Simulator 3 confirm effectiveness of the proposed method. Copyright © 2012 John Wiley & Sons, Ltd.