Both oxycodone and fentanyl are opioids, and there is cross-tolerance between different opioids to some extent. This means that if you have developed a tolerance to one opioid, you may require higher doses of another opioid to achieve similar effects.
However, it's essential to understand that fentanyl is vastly more potent than oxycodone and many other opioids. While cross-tolerance exists, this does not mean that a person who can tolerate a high dose of oxycodone can safely take a similarly high dose of fentanyl.
Fentanyl is estimated to be about 50 to 100 times more potent than morphine and about 50 times more potent than heroin, and even small amounts can cause a fatal overdose. Due to its potency, there's a very narrow margin between a therapeutic dose and a lethal dose, which is often referred to as a narrow therapeutic window.
It's also worth noting that many overdose deaths have occurred due to illicit drugs being contaminated or adulterated with fentanyl without the user's knowledge. Even if someone has a high tolerance to another opioid, it doesn't guarantee safety when consuming an unknown substance or using fentanyl.