Negative amount on box 21, T4A(OAS)
If it shows a negative amount on box 21, you should enter zero on box 21 of T4A(OAS) slip in the software as per CRA's instruction.
The amount is net benefits paid, including a net amount of any allowance, allowance for the survivor, or guaranteed income supplement you received in the year. It is always tax-free and adjusted by Service Canada.