Most osage has dark internal splits in it that we call wind shakes that might make cutting boards out a piece iffy. If you drop the tree and there are no star-burst looking brown places in the core of the trunk it should make good lumber.
Cut like you would any other lumber, paint the ends and stack with spacers between the boards.
One thing about osage is it often has a lot of internal stress so you may have some warping of your boards.