I have no idea about the oregano oil, but I doubt it. From what you wrote, I would never give that to my rats, not even externally (they will lick it off)
Do not put anything in their water - this is never ok to do, as it may cause them to stop drinking, and they can die from dehydration
As for dark chocolate, a very tiny piece is sometimes used in an emergency to try to keep a rat's airway open until you can get them to a vet. It will not prevent resp. infections and should only be given in an emergency.
Rats are prone to resp. infections and myco flare ups.
The best thing you can do is:
1. keep them in a large interesting clean cage,
2. feed them a healthy diet
(low protein from plant sources, no added salt or added sugar or added fat, a variety of vegs and occasional healthy treats such as a piece of fruit, berries, cooked grains, cooked cereals, pumpkin seeds etc)
3. clean water free from chlorine and fluoride,
4. spend time interacting with them each day in a rat safe play area where they can run, and climb and explore and interact with you,
5. get to know them well so you notice when they are ill or not acting like themselves,
6. have a good vet
7. treat illness immediately and aggressively and for long enough that they recover from the illness (4 to 6 weeks for a respire. infection or myco flareup) ....
8. do not expose them to environments where they can catch one of the serious air born rat diseases.
9. do not expose them to smoke, etc
The best defence against illness is a strong immune system
Scents can cause illness and stress can also cause illness