short answer. don't use code O
You have an adjustment not explained earlier in this column use code O
Enter the appropriate adjustment amount in column (g). See the instructions for column (g).
if none of the other statements apply
Leave columns (f) and (g) blank
there is no code letter for worthless security check the 8949 instructions codes
https://www.irs.gov/pub/irs-pdf/i8949.pdf
