The best response, but hardest to actually do, is to leave without him. You need to recognize that you can support your partner without allowing his OCD to interfere in your life.
Of course, do not do this without first warning him that the next time his rituals get in the way of your travel plans, you’ll go without him.
Let him know that you are not going to passively support his OCD by waiting for him to finish his rituals. Make him aware that his choosing to ritualize instead of leave on time is his choice and that you will not let it interfere with your plans.
When you arrive at your destination alone, you may get questions about where your partner is. You can tell people that he had some things that he needed to finish before he could leave home and then leave it at that.