After being forced from her homeland, the witch Rowena MacLeod took refuge with a peasant Polish family that showed her a kindness she had never known. During her time with the family, Rowena grew to love their eight-year-old son Oskar who was terminally ill and did not have much longer to live. As a repayment for the family's kindness, Rowena used a spell to cure Oskar's illness and cast the immortality spell so that once Oskar was fully grown, the spell would take effect and render Oskar immortal.
While the exact procedure is unknown, the spell was set on a time delay so that it would only activate once Oskar was fully grown. While the spell prevented Oskar from aging and presumably ever getting sick, he was still susceptible to mortal wounds and could die from them like any other human. Once the spell took effect, it kept Oskar in the form of a young man for over three hundred years.