Box 18 from T4A(OAS) automatically populates the Other Income Line. The result is that the income is reported twice on the tax return form. But I can't leave it bank.

Please make sure that you are using the correct T-slip page to enter your amount. Box 18 from the T4A(OAS) should go to Line 11300 - Old age security pension. But Box 018 from the T4A goes to Line 13000 - Other income.