The pilot reported that while on a local flight he noticed a squall line approaching and elected to return to the private strip from which he had departed. During the approach, rain intensity increased, and the wind began gusting to about 15 knots. The pilot landed the helicopter, lowered the collective, and reduced power. While the main rotor was turning about 420 rpm, the helicopter suddenly rolled over onto its right side, resulting in substantial damage to the main rotor system and fuselage. The pilot reported that the dynamic rollover was likely caused by the front seat passenger inadvertently contacting the cyclic control while turning to speak with the rear seat passenger. The OH-58A flight manual notes that when carrying non-rated passengers unfamiliar with helicopter operation, the pilot should evaluate the advantages and disadvantages of stowing the copilot controls or accepting the potential hazard of leaving them installed. The Federal Aviation Administration Helicopter Flying Handbook (FAA-H-8083-21B) states, “If possible, remove front passenger flight controls and ensure all passenger personal items, such as cameras and mobile phones, are secure.” Bell Helicopter Operational Safety Notice GEN-20-50 advises that pilots emphasize avoidance of flight and engine controls during preflight briefings, especially when dual controls are installed, and further notes that removing the dual controls, if installed, remains an option to prevent inadvertent interference. The pilot reported that there were no preaccident mechanical anomalies or failures that would have precluded normal operation.