We consider a fifth generation (5G) cellular system where a base station (BS) aims at sending confidential messages to a number of mobile terminals (MTs), making sure that no information on the messages is leaked to an external eavesdropper. The system uses an orthogonal frequency division multiple access (OFDMA) scheme and we aim at maximizing the weighted sum rate of the messages under constraints on the average transmit power and the secrecy outage probability. The secret messages are separately encoded over each OFDMA sub- channel. We consider a time-sharing approach where each sub- channel can be used by many MTs for different fractions of time. We obtain a convex optimization problem and we are able to establish that in the optimal solution each sub-channel is assigned to a single user for the whole time. Then, we derive a closed-form expression for the allocated power, according to a modified waterfilling algorithm.