Is Canada OAS part of US tax?

Yes, under the treaty guidelines, this is reported on your US tax return like a Social Security payment. 

1. In TurboTax, open your return and search for SSA-1099, then select Jump to in the search results
2. On the Did you receive Social Security or Railroad Retirement benefits? screen, select Yes
3. On the Tell us about the benefits you received screen, check the first box for Social Security benefits (Form SSA-1099)
4. Enter the total Canadian CPP and/or  OAS payments you received during 2023 in Box 5
   • If you already have something in Box 5 from a previously entered SSA-1099, add your CPP/OAS benefits to the amount already there
   • In the uncommon situation where taxes were withheld from your payments, enter the total withheld during 2022 in Box 6
5. Select Continue and follow any additional instructions