AWK: Am I duplicating the comma delimiter in this code?

In your code, $0 will be the exact input line, and if it came in with comma delims then those will transfer to output line. $0 is a single field so OFS not relevant there.

In the new columns you are adding you are actually adding in the comma delimiter yourself and writing one long concatenated string of the multiple column data.  So again, one field, OFS not relevant.  OFS would be used and important if you did not hard code the commas into the print() statements the way you did, and this is preferable since it allows for easier change of the delimiter later if needed.  And may be more efficient since a lot of string concatenation isn't being done.

BEGIN{FS=",";OFS=","}(NR==1){print $0,"ACCYR","Month","ACCMTH"}(NR!=1){print $0,substr($1,1,4),substr($10,4,2),substr($1,1,4)*12+substr($10,4,2)}

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~bp

I made it one print string so as to avoid creating a new line in the output file for each one, it's a workaround. I haven't tried your above yet.

MacPro:2_Original_Data_ ADM$ awk BEGIN{FS=",";OFS=","}(NR==1){print $0,"ACCYR","Month","ACCMTH"}(NR!=1){print $0,substr($1,1,4),substr($10,4,2),substr($1,1,4)*12+substr($10,4,2)} in.txt > out.txt
-bash: syntax error near unexpected token `('
MacPro:2_Original_Data_ ADM$ 

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I can't see any missing brackets..

thanks