Silica is great for removing humidity from air. But what I want is to remove oxygen from air, so my food stays fresh longer. Any clean and easy to use substance/method for that purpose?
Yeah don't. Like many metals, finely dispersed iron can self-combust on air.
I'd suggest an air-tight packaging filled with CO2, or, if your food is susceptible to acid, N2, if at all feasible.
Industry uses additives similar to hydroquinone, mixed directly into, e.g., plastics. Plenty of them are food-safe, but I wouldn't know where to buy them if you aren't a business.