I have saved enough and just retired at 58 but still have a mortgage outstanding. Should I use my CPF to pay it off? Or should I invest it for a dividend that is higher than the interest of my OA?

Since you have enough savings, you should use your bank savings, not your CPF savings, to pay off your outstanding mortgage so that you are totally debt free. Grow your CPF savings through compounding effects at 4% in your retirement account so that you could have more retirement funds at age 65. Just my humble opinion.

I believe in having a fully paid house when you retire, so that you have a peace of mind and not have to worry about the monthly mortgages. Even if you pay off your outstanding mortgage, you still have the option to redeem your CPF plus accrued interest used if you still have excess funds. Consider your priorities and go with the decision that you're most comfortable with.