Yes, IO is a valid Scrabble word!
a large, mainly yellow North American moth of the silk moth family, with prominent eyespots on the hindwings
a priestess of Hera who was loved by Zeus. Trying to protect her from the jealousy of Hera, Zeus turned Io into a heifer. Hera sent a gadfly to torture the heifer, which then fled across the world and finally reached Egypt, where Zeus turned her back into human form
one of the Galilean moons of Jupiter, the fifth-closest satellite to the planet, being actively volcanic and coloured red and yellow with sulphur compounds (diameter 3,630 km)