Do you mean Paryushan (Shvetambar) or Dashalakshan (Digambar)? Here is the simple guidance used in Jain maryada:

• Upvaas/Chauvihar: No food at all. Oatmeal is not allowed.
• Ekaasan/Biyaasan: Allowed if it is freshly cooked once/twice before sunset, using only boiled water. Do not use leftovers, do not soak overnight, and do not eat after sunset. Avoid flavored/instant packets, honey, fruits, nuts, or additives.
• Ayambil: Only if your local maryada explicitly permits oats as a grain for ayambil. If allowed, it must be absolutely plain—cooked in water only (many maryadas also avoid salt, oil, ghee, milk, curd, sugar, and spices). Rolled/instant oats are often considered processed and are best avoided.
• General Paryushan rules (Shvetambar): No green leafy vegetables; grains like oats are not the issue—the key is freshness, daylight timing, and simplicity.
• Digambar Dashalakshan: If taking bhojan on non-upvaas days, only plain, freshly cooked, simple food before sunset. On upvaas/niyama days, no food.

If in doubt, take pachchakkhan (formal resolve) from your guru or follow your local sangh’s written maryada.