One thing I forgot to mention: the 6502 microprocessor also uses groups of 3 bits in its instructions. However, they group them in the "wrong" way, aaabbbcc, so looking at the instructions in octal doesn't help you at all.

Also, after using the Xerox Alto, which uses 16-bit words, I realized that octal is terrible. The problem is that if you're looking at two bytes in a word, the values make no sense in octal. For example, the characters "AB" form the hex word 0x4142, while "BA" forms 0x4241; the two letters are clear. But in octal, "AB" is 0o40502 and "BA" is 0o41101; the two letters turn into unrecognizable numbers.