Oxidation happens when beer picks up oxygen after fermentation — during racking, transfer, or bottling — producing wet-cardboard or, at higher intensity or longer exposure, sherry-like off-flavors over time. It's the reason "minimize splashing and headspace after fermentation" is repeated so consistently across homebrewing advice: oxygen exposure before fermentation is fine and even necessary for yeast health, but exposure after fermentation is almost never wanted.
Why some styles are far more exposed to the risk than others
New England IPA, built around fresh, volatile hop aroma compounds and typically consumed young, shows oxidation damage fast and obviously — a batch that tasted vibrant at packaging can read dull and stale within weeks if oxygen exposure wasn't controlled during transfer and bottling. A Barleywine or Russian Imperial Stout, built around a much sturdier, more slowly-evolving malt and alcohol backbone, tolerates a little oxygen exposure far more gracefully, and in a genuinely aged English Barleywine a faint, controlled oxidative sherry note is even recognized as a feature at extreme age rather than automatically a flaw.
Practical technique matters more than any single ingredient choice
Careful, low-splash transfer technique, minimizing headspace in a bottling bucket or keg, and packaging promptly rather than letting finished beer sit exposed to air for extended periods all reduce oxidation risk directly — no ingredient addition undoes oxygen exposure that's already happened.