So by definition people who get “oxy itch” are experiencing an allergic reaction. However, itching without the presentation of a rash or restricted.
There are two ways to look at this question. On the one hand, oxycodone is a codeine derivative, and ingesting codeine products, whether naturally or synthetically formed, does cause histamine to be released in the body. The body’s reaction to histamine release is the hallmark of an allergic reaction: irritation and inflammation in various parts of the body. So by definition people who get “oxy itch” are experiencing an allergic reaction.
If it was that severe, it was most likely an allergic reaction that led to anaphylactic shock, but you'd Oxycodone and Codeine are two compley different drugs.