So Oregano is a perennial herb, growing from 20–80 cm tall in the wild and is closely related to marjoram. However oregano has a much coarser, more robust flavor with a hint of thyme, and a warm aroma.(source bbc good food)
Oregano is much used as a herb in culinary dishes and tends to go well with tomatoes, aubergine and lamb.
So if we can eat it, can guinea pigs eat oregano, and if they can how much of it can they eat?

Lets take a look at its nutritional data to find out a bit more about it.
As per usual, we’re particularly interested in its calcium, phosphorus, acidic, fat and sugar content as they are the nutrients that affect guinea pigs the most.
Total Fat 4.3 g 6%
Saturated fat 1.6 g 8%
Polyunsaturated fat 1.4 g
Monounsaturated fat 0.7 g
Trans fat 0 g
Cholesterol 0 mg 0%
Sodium 25 mg 1%
Potassium 1,260 mg 36%
Total Carbohydrate 69 g 23%
Dietary fiber 42 g 168%
Sugar 4.1 g
Protein 9 g 18%
Vitamin A 34%
Vitamin C 3%
Calcium 159%
Iron 204%
Vitamin D 0%
Vit B-6 50%
Vitamin B-12 0%
Magnesium 67%
source google
As you can see Oregano contains a huge amount of calcium. Without looking at those other nutrients, that completely rules out Oregano as a food for piggies, they just could not cope with a food with that amount of calcium in it.
So do not feed Oregano to your guinea pigs if you can help it.