don't get confused by the word "orthogonal".
Here's what happens in simpler (?) words ... and based on understanding of the Sinc function.
if you space the subcarriers from one another by any amount equal to the reciprocal of the symbol period of the data signals, the resulting sinc (sin x/x) frequency response curve of the signals is such that the first nulls occur at the subcarrier frequencies on the adjacent channels. The subcarriers all have an integer number of cycles within the symbol period. With this arrangement, the modulation on one channel won’t produce intersymbol interference (ISI) in the adjacent channels.
hope this helps