Hi,
Yes i agree that some rectification and DC filtering is a good idea, but one thing you have to watch out for is not to go too high on the filter capacitance or it could cause the relay to open too slowly when it is turning off. This could cause bad arcing between contacts depending on the application. So use a capacitor value that wont cause the relay to open too slowly, or add a parallel capacitor to help discharge the capacitor faster.
So this means a resistor in series with the op amp output, a diode such as 1N5817, a capacitor to ground with parallel resistor, then another resistor from the capacitor to the base of the transistor. A little trial and error should get you there or else we could Spice this thing. The coil itself holds energy for a short time too so that might help filter a little also.