What should a person suffering from obsessive-compulsive disorder who missed prayers during the period of their OCD do to make up for these prayers, when they are currently unable to make them up?

Giving in to obsessive thoughts and responding to them causes a lack of concentration. The cure for obsessive thoughts is to turn away from them after seeking refuge in Allah. It is not permissible to abandon prayer under any circumstances. Whoever has abandoned it must repent to Allah and make up for the prayers they missed, if they know the number of missed prayers, or make up what they predominantly believe is sufficient. Nothing can substitute for making up missed prayers.