There are several words which begin using the four letters OCTA...

Octagon, octagonal, octagons, octal, octane, octave, octaves

Most of them refer to shapes, math, or music. There are a few others that sound like they might start with the letters OCTA but actually don't; for example, octopus.

Octal is a different way of representing numbers. Standard numbers up to seven are the same in octal, but others are different; for example, fifty becomes sixty-two.

Octaves relate to music. According to the manual for "Personal DeskMate 2", an octave is a "series of six whole steps, such as the notes between C and C".