This work investigates the impact of techniques to improve the channel estimation in Orthogonal Multi Carrier CDMA (Code Division Multiple Access) Systems: MC CDMA (Multi Carrier Code Direct Multiple Access) and MC DS CDMA (Multi Carrier Direct Sequence Code Division Multiple Access) in frequency-selective channels. Two types of guard interval are considered, the zero padded (ZP) and cyclic prefix (CP) the most used in the literature. Initially, the channel estimation is assisted by pilot symbols and based in the minimum mean square error criterium. Analytical expressions for the mean square performance of the conventional estimator and two improved channel estimates: applying a projection matrix and applying a initial restriction are presented. In blind channel estimation two techniques are investigate for ZP MC DS-CDMA systems and alternatives methods are proposed. The first is based in subspace method and power techniques. The second is based in correlation matching and square minimization of the cost function; that is defined by the square Frobenius norm of the error matrix. This error matrix is obtained with matching the correlation matrix against its sample average estimated from the received data. The simulated results revel that the proposed assisted and blind methods achieve good performance with softer computational complexity in both systems.