The pills are 10 years old OP. If not "properly" stored, they begin to oxidize and lose potency. Chemical structures become unstable due to exposure and other various circumstances over long periods of time. Nothing lasts forever.
Being that the chemical composition of this oxycodone has degraded over the years, it makes complete and evident sense that they aren't working as well as fresh pharmaceutical pills. The chemical structure of oxycodone is engineered in such a way that it is meant to bind to your opiod receptors, sort of like a lock and key. The drug has an affinity for these receptors, and travels through the blood until it reaches them and locks in. Once the chemical becomes psychoactive you then begin to feel the effects of the drug (the onset) until it reaches peak blood plasma levels.
So it stands to reason that older drugs whose chemical composition has become destabilized will no longer fit that lock and key.
There are quite a few more technical details on what happens but I'm trying to say that expired pills will work they are just weaker and unstable, unpredictable. Still though, I have found ancient pills before stored with care and gotten some great highs.
Also, oxycodone has a short half life.