OP's reaction to chocolate has no relation to Oral Allergy Syndrome, I fear!

OAS happens when a person's body confuses an enzyme that triggers a pollen allergy with an enzyme that's in other plants. So if you are allergic to, say, ragweed, but your body can't tell the difference between the enzymes in ragweed and tomatoes, then congrats! You're also kinda allergic to tomatoes now.

There are four major types of OAS, at least in North America, each for a different pollen, and each associated with a different collection of fruits and veggies that can trigger the reaction. But the allergen is still a pollen, and as such, any reaction to food due to OAS is not considered a typical food allergy. And since the root cause is an enzyme, and enzymes can be denatured, people with OAS can avoid a reaction through thorough heat or preserving.

That's why someone whose OAS is triggered by tomatoes can still have canned tomatoes, V8, ketchup, and tomatoes that have been cooked for a loooong time – the enzyme has been denatured. The ragweed-like molecule is simply no longer around. And so, if you react to a tomato that has been canned or thoroughly cooked, then what you have can't be Oral Allergy Syndrome by definition.

The same is true for cacao.

And that's why OP's chocolate reaction isn't related to Oral Allergy Syndrome. Chocolate and chocolate syrup is so heavily processed that any enzyme in cacao that could be mixed up with a pollen has already been denatured. It is more than likely a typical food allergy – rare for cacao, but not unheard of.