Hexadecimal (base 16), decimal (base 10), octal (base 8), and binary (base 2) are the most commonly used numeral systems in engineering and computing. Therefore, Excel has provided functions to convert numeric values to and from each of these systems.
The Excel Dec2Oct function converts a decimal number into an Octal (Base 8) number.
The syntax of the function is:
Where the function arguments are as follows:
number    The decimal 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 [places] is omitted, the returned octal uses the minimum number of places. 
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 Dec2Oct function.
Formulas:

Results:

Note that, in the above example spreadsheet, the negative octal number in cell A4 is represented by two's complement notation.
Further details and examples of the Excel Dec2Oct function are provided on the Microsoft Office website.
If you get an error from the Excel Dec2Oct function this is likely to be one of the following:
#NUM!   
Occurs if either:

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