By this definition, an octave can be considered harmony. If that's the case however, if one were to play a diminished chord, a tritone or even a dominant 7th chord, it wouldn't be considered harmony, as those intervals and chords don't exactly "produce a pleasing effect" in most ears.

What is octave harmony?

Octave, in music, an interval whose higher note has a sound-wave frequency of vibration twice that of its lower note. ... In harmony and in instrumentation, a note paired with its octave is said to be doubled. Melodic doubling in octaves is ubiquitous in all types of instrumental music.

What is harmonizing in singing?

What Is Vocal Harmony? Singing harmony involves supplementing a vocal melody with additional notes that fit the underlying chord structure.