In this paper, an effort is made to analyze the integration of direct detection optical orthogonal frequency division multiplexing (DDO-OFDM) with wavelength division multiplexing (WDM) to reach high data rates of 1.050 Tb/s over 3600 km single mode fiber (SMF). The 1.050 Tb/s signal is generated by multiplexing 30 OFDM signals with 35 Gb/s for each OFDM. The performance of the system is studied by measuring the optical signal to noise ratio (OSNR) of each WDM channel;signal to noise ratio (SNR);and bit error rate (BER);while analyzing the constellation diagram of all users. Also, the relationship between the OSNR and BER is studied and it is noticed that as the OSNR increased, the BER decreased. As can be seen form the results, as the transmission distance increased the BER increases and to keep the BER less than 10-3 we need to increase the OSNR.