In a binary number, the smallest addressable value is called as a bit.
In a decimal number, the smallest addressable value is called as a digit.
In an octal number, what is the smallest addressable value called as?
No matter what base system you use it is called a digit.
So if it is not clear from the context and you need/want to emphasize that it is a digit of a number in base 8 representation then call it octal digit.
Note: Also "bit" is just short for "binary digit".
which all fit quite nicely.
Having said that, having been a programmer for over 30 years I have never come across any of these words being used in this sense.