For a $3,000 monthly pension payment ($36,000 annually), a portion is taxed at 10% and a portion is taxed at 20%. A retro-payment of $3,500 would be taxed at 20% because it falls in the $30,000 to $60,000 income range.
When you file your tax return, your total OMERS income, including the retro-payment, is factored in with the tax paid for the year. If your tax paid is in excess of what is required on your income from all sources (e.g., CPP, OAS, RRIFs, etc.), you would get a refund from CRA.