What is smallest addressable value in an octal number called?

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".

According to the Obsolete and unusual units section of the Units of information wikipedia page, an octal digit (3 bit word) has been variously described using the terms:

• tribit, triad, triade, and tribble.

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.