Where do i enter T4A(OAS)?

Click on the INCOME tab at the top,

then click on the link for T-slips in the drop down list

then on the t-slip profile page check off the T4A(OAS) slip and click continue until you get to that slip.

it was helpful but when I do this the end result shows income for social assistance

The amounts from Box 21 of the T4A(OAS) are considered social assistance, they are the senior supplements. The amount in Box 18 is considered pension income.