Different Enemy, Different Tool
The two sachets target two different molecules. A desiccant — silica gel, activated clay or calcium chloride — adsorbs or absorbs water vapour, pulling the relative humidity down so the reactions that need moisture (rust, mould, mildew, caking) cannot proceed. An oxygen absorber, usually iron-based, chemically binds the oxygen in the sealed headspace, driving it toward zero so the reactions that need oxygen (metal oxidation, fat rancidity, aerobic bacteria and insects) are starved. Neither does the other’s job: a desiccant leaves oxygen untouched, and an oxygen absorber leaves humidity untouched.
So the question is simply: what actually destroys your product — moisture or oxygen? Corrosion of machined parts, condensation in a container, damp electronics, clumping powders and mould on textiles are moisture problems, and a desiccant is the answer. For industrial and export packaging, moisture is the dominant threat by far, which is why desiccants are the workhorse of protective packaging. Oxygen absorbers come into their own for long-life food, oxygen-sensitive chemicals and certain bright metals stored airtight.