Quality of service (QoS) aware resource allocation for the uplink of a decode-and-forward relay assisted Orthogonal Frequency Division Multiple Access (OFDMA) based cellular system is investigated. Incorporating relays in the system improves the cell-edge coverage and the system throughput. Resource (relay, subcarrier and power) allocation problem is formulated with the objective of maximizing the total system throughput subject to the satisfaction of user QoS requirements and individual total power constraints of the users and relays. The throughput of each end-to-end link is modeled considering both the direct and relay links. Due to non-convex nature of the original resource allocation problem, the optimal solution is obtained by solving a relaxed problem via two level dual decomposition. The performance of the proposed scheme is evaluated in the scenarios based on LTE-A network model. Numerical results reveal that the proposed scheme guarantees each user's QoS satisfaction at the expense of a slight degradation of the system throughput.