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This letter investigates secure transmission for a cooperative non-orthogonal multiple access (NOMA) network consisting of multiple near users (NUs), a far user (FU) and an untrusted relay. In each slot, the base station serves an opportunistically scheduled NU along with the FU by using NOMA. We design a cooperative jamming based secure transmission strategy to prevent the information leakage of the NU and the FU against the untrusted relay. A novel NU selection scheme is proposed to enhance the secrecy performance, while guaranteeing the scheduling fairness among all NUs. Considering that all channels of NUs experience independent but non-identically distributed Rayleigh fading, the closed-form average secrecy rate and its scaling law are theoretically derived for the FU and the scheduled NU. Finally, simulations are provided to validate the secrecy enhancement and the scheduling fairness of the …