The Excel Oct2Bin function converts an Octal (Base 8) number into a Binary (Base 2) number.
The syntax of the function is:
Where the function arguments are as follows:
number    The octal number that is to be converted to binary. 
[places]   
An optional argument, which specifies the number of characters that you want the returned binary number to have. If this is greater than the minimum, the binary number will be padded out using leading zeros. If omitted, the returned binary 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.
+ Display Octal and Binary Summary:
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 
For further information on the octal numeral system, see the Wikipedia Octal Page
The Binary (Base 2) Numeral System uses the digits 0 & 1.
The following table shows the first 8 binary values, along with the equivalent decimal values:
Binary  1  10  11  100  101  110  111  1000 
Decimal  1  2  3  4  5  6  7  8 
The following spreadsheets show five examples of the Excel Oct2Bin function.
Formulas:

Results:

Note that, in the above example spreadsheet, the negative octal and binary numbers in cell A4 are represented by two's complement notation.
Further details and examples of the Excel Oct2Bin function are provided on the Microsoft Office website.
If you get an error from the Excel Oct2Bin function this is likely to be one of the following:
#NUM!   
Occurs if either:

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