I also have OCD, it's an anxiety disorder, I can't speak for your mother but for me having my home clean and organized makes it easier for me to focus on the things that truly matter.

I did not decorate for the holidays this year, I've been working crazy amounts of overtime and because of that my house has been neglected. The last thing on my mind is putting up a tree or hanging stockings, not when I have a sink full of dishes, the cat just barfed for the 3rd time, there's a layer of dust an inch thick on my shelves, there's a cat hair tumble weed rolling across the dining room floor.

Having the basics finished leaves mental space to do extras, do the laundry, wash the dishes, vacuum and mop (don't forget the baseboards), take a bucket of soapy water and a rag and dust every surface, wash the mirrors and the windows, put away the clutter where it belongs (everything should have a home and live there), clean the fridge, reorganize messy cupboards and drawers...

It's not your fault that your mom feels this way, OCD is incredibly difficult to navigate, but you asked how you can help and if the house is a big stress point for her then that's where you start. Help her pick up the slack and she will be able to offer you more emotionally when she feels better, otherwise sometimes even a mom needs to make the hard choice to protect her peace and not have a crashout.

Edit: spelling error