Yes, OCTAVO is a Scrabble word!

Letter values in Scrabble US: O 1, C 3, T 1, A 1, V 4, O 1.

Is it a valid word?

OCTAVO is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the size of a book whose pages are made by folding a sheet of paper three times to form eight leaves

Also: 8vo.