Car ramps are safe to use for oil changes.
When using car ramps to raise your car up or vehicle up to change the oil make sure to set your parking brake or emergency brake and chock the wheels.
Put a wheel chock behind each of the rear wheels to avoid the possibility of the car or vehicle rolling back although the emergency brake or parking brake should hold.
You just never know and it's better to be safe than sorry.
Drive your car or vehicle up on a good set of car ramps and then put the vehicle in park and if it's a standard transmission put the transmission in reverse.
Put the parking brake on and chock the wheels and it should be very safe as long as the car ramps are in good conditions.