Re: How best to replace old skip sheathed wood shakes with asphalt shingles?
First call you home insurer see if you get a large discount if you change to a level 4 impact resistant asphalt shingle! on all future payments - its a big deal. Also they should reduce the costs once you no longer have shakes as you pay higher rates with wood roofs.
OSB is cheaper then plywood and is installed on thousands and thousands of roofs with out issues. the other posts issues are rare.. See what your code allows as you are simply overlaying spaced decking maybe 3/8" will be allowed. the spaced decking is quite strong! It performs very well. be sure if you use it that you get the roof underlayment on prior to rain if it gets wet let it dry a few days before covering it up. Be real sure you consider additional roof venting because shakes on spaced decking usually have no other roof venting. Consider installing a whole roof ridge cap vent when shingling and soffit or eve vents to allow air into the attic. Also consider the non roof felt underlayment it is less expensive then felt and 6 moths to 1 year exposure to sunlight . Lays flat, resists wind, and is getting very popular i use GAFs product.
Any an all of my comments are just my opinion and not to be confused with facts.