If the ramps can support the car safely (they not about to bow in from weight like a Rolls Royce or Ford Excursion or truly massive thing above 4500lbs or so) and the clearance is enough for what you need to do, then it is very safe as the car's entire suspension evenly distributes the weight and load across whole axle, and two tires points.. again, the weight ends up on the ramps. Jack stands could help, or if you have old tire rim or two to add to under, just in case.
Masonry products like bricks or concrete or cinder blocks are discouraged as they may crack, and a friend on another car forum had a friend lost his life when car fell on him, not properly supported (what a way to go..) but it's been seen done. You can't have it supported too much.
For something like an oil change, if it gets high enough, you would "probably" be OK with a ramp.
You could do an OC with a tire up on a curb, most the time. You better have an oil catch pan, though. If not, it is beyond environmentally irresponsible and you shouldn't be doing the OC, though we all likely know one person that has done something like just left the oil there, kaybe even by a storn drain, and gotter away wit it. Not cool.