Net flaming that occurs on Online Social Networks (OSNs) has become a serious problem. Net flaming occurs when some news is spread on OSNs and users react strongly to it. In OSNs, however, net flaming can occur even though there is nothing particularly wrong with the news itself that is spread. In this paper, we use a network oscillation model to analyze user dynamics caused by news propagation in OSNs, and show that net flaming may be caused by user comments added to the news during the propagation process. Based on these observations, we discuss countermeasures to suppress net flaming.