The Excel Hex2Oct function converts a Hexadecimal (Base 16) number into an Octal (Base 8) number.
The syntax of the function is:
Where the function arguments are:
number    The hexadecimal number that is to be converted to octal. 
[places]   
An optional argument, which specifies the number of characters that you want the returned octal number to have. If this is greater than the minimum, the octal number will be padded out using leading zeros. If omitted, the returned octal uses the minimum number of places. 
Note that the supplied number argument must not be more than 10 characters (40 bits) long. The most significant bit of this value denotes the sign of the number and the remaining 39 bits denote the magnitude. Negative numbers are represented using two's complement notation.
It should also be noted that, as hexadecimals use the numbers 09 and the characters af, they should be enclosed in quotation marks when they are supplied to an Excel function. (e.g. The hexadecimal 11a should be input as "11a").
+ Display Hexadecimal and Octal Summary:
The Hexadecimal (Base 16) Numeral System uses the digits 09 and the characters af.
The following table shows the first 32 hexadecimal values, along with the equivalent decimal values:
Hexadecimal  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f  10  11  12  13  14  15  16  17  18  19  1a  1b  1c  1d  1e  1f  20 
Decimal  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32 
For further information on the hexadecimal numeral system, see the Wikipedia Hexadecimal Page
The Octal (Base 8) Numeral System uses the digits 07.
The following table shows the first 16 octal values, along with the equivalent decimal values:
Octal  1  2  3  4  5  6  7  10  11  12  13  14  15  16  17  20 
Decimal  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16 
The following spreadsheets show five examples of the Excel Hex2Oct function.
Formulas:

Results:

Note that, in the above example spreadsheet, the negative hexadecimal and octal numbers in cell A4 are represented by two's complement notation.
Further details and examples of the Excel Hex2Oct function are provided on the Microsoft Office website.
If you get an error from the Excel Hex2Oct function this is likely to be one of the following:
#NUM!   
Occurs if either:

#VALUE!    Occurs if the supplied [places] argument is nonnumeric. 