There are literally dozens if not hundreds of types of Oputia cacti. Some are covered in spines, some have none, and most are in between. You should always scrape them off before you give them to your tortoises as a snack. If your cactus has any spines that are stiff you really should remove them.
Through not very scientific experiments I have deduced if I can get pricked or hurt by something, then your tortoises may also. I've had mixed results with my sulcata, some love it and devour it, others have no interest. Another issue is if your torts eat it and like it, they may devour it before it can even root. If you have some that are expendable, you could try to scrape off the spines, and then plant it and see if it roots. If it does, you can regularly check and scrape the spines and let your torts live around it, I've had good results with this before I found some without spines.
To be safe, You should plant it in a secure place, they grow and reproduce fast. then you can experiment with the pads. Or try to find some that have no spines. If you like the way it looks, and want it in the enclosure for aestetic purposes, you could just plant it high or surrounded so your torts can't get to it, then you can scrape it and monitor the intake.
I'm sure you know with almost everyting with tortoises:
better safe than sorry.
hope this helps.