If I buy a tenanted property at a tax sale, do I have to follow normal RTA notice rules to have the tenant leave?

Yes - buying a property through a tax sale does not exempt you from the standard rules that apply to any Ontario landlord seeking to end a tenancy. If a genuine tenant is living in the property, that tenancy generally continues after the tax sale, and you step into the landlord role along with the obligations that come with it, including the requirement to follow the Residential Tenancies Act's normal process and valid grounds if you want the tenant to move out.

There is no special fast-track or exception simply because the property changed hands through a tax sale rather than an ordinary purchase; treating it that way risks putting you offside with tenant protection rules and creating legal problems of your own, on top of whatever other issues came with the distressed purchase itself. If ending the tenancy is important to your plans for the property, factor in the time and process that normal landlord-and-tenant rules require, rather than assuming the tax sale itself resolves the occupancy question.

Speak with a lawyer about your specific obligations as the new landlord before taking any steps toward ending the tenancy.