In a downlink non-orthogonal multiple access (NOMA) system, the reliable transmission of cell-edge users cannot be guaranteed. On the other hand, the presence of eavesdroppers can severely threaten the secure transmission. Thus, a two-user NOMA system assisted by a multi-antenna decode-and-forward (DF) relay is considered in this paper, and a two-stage jamming scheme, FDJam, is proposed to ensure the secure transmission of NOMA users. In the FDJam scheme, using full-duplex (FD), the relay transmits the jamming signal to the eavesdropper while receiving confidential messages in the first stage, and the base station (BS) generates the jamming signal in the second stage. Furthermore, we eliminate the self-interference and the jamming signal at the relay and the legitimate node, respectively, through relay beamforming. To measure the secrecy performance, analytical expressions for secrecy outage probability (SOP) are derived. Simulation results are presented to show the effectiveness of the proposed scheme.