Are hot stars like O-type stars entirely composed of helium? Hot stars like O-type stars show no hydrogen in their spectra. Does this mean they are made entirely of helium?

Any explanation would be really helpful.
The lines that appear in a stars spectrum mainly reflects its temperature not its composition, see here

O-type stars start out with the same sort of composition as other stars, that is they are mainly H and He (approximately 75% and 25% by mass) with traces heavier elements.