Per IRS Publication 915:
Under income tax treaties with Canada and Germany, social security benefits paid by those countries to U.S. residents are treated for U.S. income tax purposes as if they were paid under the social security legislation of the United States.
In other words, if you're a U.S. resident, you'd enter your Canadian CPP and OAS benefits as though you'd received a Form SSA-1099 from the Social Security Administration.
The U.S.-Canada tax treaty also applies to the Québec Pension Plan (RRQ). Note that the agreement only addresses government-issued Social Security benefits, not money received from private Canadian pensions.