I imagine you've probably resolved this by now, but have you got any pictures of the board?
Woodglue and sawdust is about the quickest, but will not darken down with the rest of the wood if you apply and oil or stain as it is no longer porous. If it isn't a particularly heavy use item (and depending on the cracks) then a colour matched wax to fill the cracks is quite good as you can blend the colour a bit to match your finish. Otherwise possibly a bit of alcohol dye in some epoxy if they're biiiiiig cracks (though this will have the same issue of not changing colour with your applied finish, you have a little more control when it comes to the colour.