If you entered the OAS pension amount shown in Box 18 on a T4A slip, it will incorrectly post to Line 130 - Other Income.
You will need to delete this entry by going to the Income Tab > T-Slips > T4A-Pension, Retirement, Annuity > while on the T4A Summary page, choose to "delete" the slip entered in error.
Now you can re-enter your OAS on the correct screen - T4A(OAS)