But what's to indicate that the function is addition?
neither is a function really. they are different ways of configuring digits to represent values.
octal uses 8 as its base, meaning it has 8 single digit values, 0-7. decimal uses 10 as its base, meaning it has 10 single digit values, 0-9.
with octal, the 8th positive integer (9th non negative integer) is also the first double digit value, 10. the same 0-7 principle applies, and after 17, you go to 20 and so on.
decimal is what we use. when you get to the actual value of 10, the digits you use are 10.