We consider the problem of the joint allocation of power and cyclic prefix (CP) duration to maximize the system capacity in orthogonal frequency division multiplexing (OFDM). A closed form solution to the problem does not exist and the exhaustive search of the optimal values is practically unfeasible. Therefore, we propose a sub-optimal iterative power allocation algorithm jointly with the CP adaptation to see whether capacity improvements can be obtained w.r.t. the conventional choice of a uniform power distribution over the sub-channels and a predetermined long CP value. Numerical results show that via power and CP adaptation significant improvements can be obtained.