Offering a castrated ram as an Ud-hiyah is permissible, because this is not a blemish but it is better for the flesh of the animal. It was authentically reported that:
The Prophet (peace be upon him) sacrificed two white, castrated rams with big horns.
This is the wording of
Al-Hakim as mentioned in the book "Al-Muntaqa".May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.