The oat, sometimes called the common oat, is a species of cereal grain grown for its seed, which is known by the same name (usually in the plural, unlike other cereals and pseudocereals).

While oats are suitable for human consumption as oatmeal and rolled oats, one of the most common uses is as livestock feed. (source)

So can guinea pigs eat oats?

Let’s take a look at their nutritional data and find out more.

In particular, their acidic, water, sugar, fat, salt, calcium, and phosphorus content is of most interest as far as guinea pigs are concerned.

Nutritional value per 100 g (3.5 oz)
Energy 1,628 kJ (389 kcal)
Carbohydrates
66.3 g
Dietary fiber 10.6 g
Fat
6.9 g
Protein
16.9 g
Vitamins
Thiamine (B1) (66%) 0.763 mg
Riboflavin (B2) (12%) 0.139 mg
Niacin (B3) (6%) 0.961 mg
Pantothenic acid (B5) (27%) 1.349 mg
Vitamin B6 (9%) 0.12 mg
Folate (B9) (14%) 56 μg
Minerals
Calcium (5%) 54 mg
Iron (38%) 5 mg
Magnesium (50%) 177 mg
Manganese (233%) 4.9 mg
Phosphorus (75%) 523 mg
Potassium (9%) 429 mg
Sodium (0%) 2 mg
Zinc (42%) 4 mg
Other constituents
β-glucan (soluble fibre) 4 g

source wikipedia

As you can see, oats contain a hint of salt, a huge amount of phosphorus and acidic content, quite a bit of calcium and fat.

This means that guinea pigs can only eat a small portion of oats as a treat. They are not a good food for them to eat due to the amount of phosphorus and acidic content that they have.