I believe these two topics are often misunderstood. Let's make it more clear.
-OCD starts with obsessive thoughts.. IF your way to deal with these thoughts includes compulsive behaviours then you have OCD.
Now what's phobia?
Phobia is an extreme irrational fear od specific object or situation. It is also irrational fear and can include over checking or avoiding places that seem dangerous to the person who has a phobia.
Then what's the difference, aren't both of them irrational fears?
Well yes, BUT there's an EXTRA dimension to OCD
A person with OCD usually has a strong feeling that then need to carry out their compulsions or else some dreadful consequence will ensue and almost always then feel that they must do their compulsions in a certain way, like a RITUAL.
Let's say you are afraid of 🕷️
-If you are afraid of spiders and need to check every room for them, you have a PHOBIA.
-If you feel that letting a spider be in the room is likely to bring bad luck or harm your family and you also check the room at the same time each day, you have OCD.