Doppler frequency offset, Doppler spreading, and multipath effects are induced by the rapid movement of Low Earth Orbit (LEO) satellites, which collectively result in two-dimensional (2D) time-frequency fading within Non-Terrestrial Network (NTN) systems. Consequently, this leads to a degradation in the signal-to-noise ratio (SNR) uniformity and deep fading across the time-frequency grid in Orthogonal Frequency Division Multiplexing (OFDM) systems, which significantly impairs the bit error rate (BER) performance. The Orthogonal Time Frequency Space (OTFS) scheme is capable of addressing 2D time-frequency fading but at the cost of increasing the complexity of the receiver. In this work, an orthogonal transform-assisted OFDM (OTA-OFDM) scheme is proposed, which is based on orthogonal transforms to map data symbols into the time-frequency grid, effectively spreading the data symbol energy throughout the time-frequency domain. Simulations within a 400 MHz NTN system indicate that at a BER of 10-3, OTA-OFDM outperforms OFDM with SNR gains of 3.73 dB and 1.92 dB in NTN-TDL-B and NTN-TDL-D channels under QPSK modulation. It also obtains 3.24 dB and 1.54 dB SNR gain respectively under 16-QAM modulation. In addition, OTA-OFDM achieved an 93.33% reduction in receiver channel estimation and equalizer module complexity while matching the performance of OTFS.