Octaves are consonant or dissonant depending on the timbre of the sound. For sounds produced by harmonic instruments, like a vibrating string or air vibrating inside a long tube, the sound is a superposition of waves whose frequencies are integer multiples of a fundamental frequency. Then, playing two sounds an octave apart will match exactly all these frequencies and it will sound nice. But there are other instruments (not privileged in the western music tradition) whose timbre is not composed of integer multiples of a fundamental frequency; and in these instruments octaves sound very dissonant.
You can argue that the octave is a "basic phenomenon" inasmuch a vibrating string is basic. Yet, from the point of view of a person who uses a synthesizer, the octave has nothing special with respect to other intervals.