Oats (Avena sativa) are a good source of soluble dietary fiber (especially β-glucan), and also contain meaningful amounts of ferulic acid, iron, selenium, manganese, thiamin (vitamin B1), and zinc. Oats are also a dietary source of melatonin and the lignan enterolactone. Avenanthramides, polyphenols found in oats, have been shown to exhibit anti-inflammatory and anti-itch activity in the skin.
Consumption of oats or β-glucan has been shown to reduce cholesterol, improve colon health, offset the increased risk of upper respiratory tract infection caused by stressful exercise, and reduce high blood pressure in obese subjects. High consumption of whole grains has been associated with lower risk of colon cancer.
Whole grain consumption has also been shown to be associated with greater insulin sensitivity and lower body mass index in adolescents, especially among the heaviest. A diet containing 10% oat lipids has been shown to reduce the incidence and size of carcinogen-induced liver tumors in laboratory rats.

Breast cancer-related effects of eating oats

Oats in diet

Diets rich in whole grains and/or soluble fiber have been found to be protective against breast cancer in some studies. Consumption of oats increases bowel motility, which has been linked to lower breast cancer risk since it increases estrogen excretion.
A 2018 study reported that Finnish women with high oatmeal consumption had a lower risk of breast cancer. A 2019 Danish study reported that high consumption of oats (oatmeal, muesli) before diagnosis was associated with lower all-cause mortality in women with breast cancer.

Melatonin

Oats contain some melatonin, which protects against breast cancer in several ways. Melatonin protects against ER+ breast cancer by reducing aromatase activity within the breast, thereby decreasing estrogen production. Melatonin has also been found to reduce triple negative (ER-/PR-/HER2-) breast cancer growth, proliferation and migration in cell and animal studies. In addition, melatonin has been shown to reduce the cardiotoxicity associated with Adriamycin (doxorubicin) chemotherapy.

Selenium

Oats are also a good source of dietary selenium, which is associated with reduced risk of breast cancer.

Additional comments

Oat groats are whole oats that have been hulled (to remove their inedible hulls). Steel-cut oats are groats that have been cut into two or three pieces. Rolled oats are made by steaming oat groats and then rolling them into flakes. All three are good choices.
Below are links to recent studies concerning this food and its components. For a more complete list of studies, please click on oats.