Can new sheathing be laid over my original roof decking?
So your existing roofing is either wood shingles, wood shakes or tile ( clay, concrete, etc.) These types of roofs can span across gaps in the roof sheathing. 

Your new roof is going to be composition shingles that need solid sheathing. 

Yes, it is quite common to add solid sheathing over the top of the spaced sheathing. The added weight will be off-set by removing the heavier existing tile roofing and the heavier wood shingles and shakes (when they are soaked with water).

Here are some precautions: 1) make sure the new roof sheathing is a minimum of 1/2” thick. Roofing manufacturers require a minimum of 1/2” for “holding power and withdrawal”...blow off. 2) Increase the height of your perimeter flashing by at least 1/2” to compensate for the additional height due to the thickness of the roof sheathing. (You want the same lap over the siding.)