Oatmeal in little packages from the store are not true "long term" although they will generally last longer than their expiration date shows.
A little oat background helps to understand how to store it:
While feed oats are left in their hull and may be "crimped" for ease of digestion, any oat kernel will begin to break down within a few days if the germ is exposed to air. Whole oats keep longer than crImped. Neither do well past one season because the germ is loaded with oil. Next to corn, oats have about the highest lipid content of any commercial grain. Remember, fats will not store well long term unless frozen or stabilized.
Oats for human consumption are hulled then "kilned" to stabilize by evaporation the oils. The remaining groats are sorted into sizes; whole, coarse, medium, fine. These pieces are what's referred to as "steel cut." They may be rolled flat or ground into flour. Depending on the piece size going into the roller, they may become "instant" (smallest pieces), "quick" or "baby" (medium pieces), and "regular rolled" (larger pieces). Smaller, thinner, flakes cook faster. While the germ is not intentionally removed, the smallest pieces that broke off in processing (which become the instant) inherently contain less germ.
Bottom line, regular rolled oats are healthier but slightly harder to store. All rolled oats should be kept oxygen and moisture free, and stored cool.
Mine are bucketed in CO2 (which is naturally anhydrous) with silica gel desiccant packets. I have both regular and quick oats buckets, plus some steel cut in cans. I expect to see the shortest shelf life from the steel cut groats due to the higher lipid content. Rolled and quick make sense to store because they keep well and cook quickly. Energy availability to cook with could be a factor unless you are in cold country using the same heat source to cook and heat with (ie: woodstove).