Obviously, you know that your OCD makes you extremely cautious, and it’s great that you’re aware that your DS isn’t actually a major problem here. But still, I wonder whether there’s a way to make him more aware of being careful around the baby, to give you some extra peace of mind? Do you think that you and DH could have an age-appropriate chat with DS to help him understand and feel invested in being careful around your DD?
You might tell him that mums and dads are there to look after their children: his mum, his dad, and you are all there to look after him so that he can be happy and healthy. And now you also have to do the same for his baby sister. Ask him to think of some things that you and Dad need to do for baby sister, that you don’t need to do for him because he’s a bigger boy now. Then say that there are lots of things that you can help him with that his baby sister doesn’t need yet (maybe things like getting him ready for school, helping him with reading or writing, making sure he doesn’t eat too many sweets, playing football with him…). The idea is to get him to the conclusion that families need to do different things for big children than they do for babies.
Explain to him that, when babies are small, it’s easier for them to get hurt by accident.
And sometimes, mums or dads get extra worried because they’re learning how to look after the new baby. Tell him that everyone can help by all learning together how to keep the baby healthy and happy. Say that, until baby’s a bit bigger, it’s important that we’re all gentle with her. It’s also important that we try to keep coughs and colds away from her, if we can, because she’s so small. Help him think of some ways you can all do these things - moving away from the baby if you need to cough or sneeze, washing hands after blowing your nose, and being extra-specially gentle when you touch or hold the baby, for example. Tell him how happy you are that he is there to learn with you about what the baby needs. Tell him that he will be able to help his sister grow up into a little girl who is lovely, kind, and helpful - just like her big brother.
Obviously, you don’t want to make him worry that something catastrophic will happen to his sister. But I wonder whether framing this as something that you all learn about together, and something that’s temporary while the baby is very small, might make him feel involved without scaring him, and therefore make him more likely to remember to be careful?
I don’t know whether you think any of that would be helpful, or whether you think it would be appropriate for your DS. But even if not, maybe it could inspire some other ideas for giving him a sense of ownership over being careful around the baby - letting him understand the rationale behind it, rather than just seeing it as something he’s being told to do?
Good luck, OP. I hope you can access treatment for the OCD very soon, and that it is helpful. And congratulations on your lovely family!