Composition of desiccant
Yes, calcium oxide can be used as an desiccant.

The following quote is an excerpt from this site:-


  Calcium oxide is calcinated or recalcinated lime having a moisture adsorptive   capacity of not less than 28.5% by weight. The
  distinguishing feature of calcium oxide is
  that it will adsorb a much greater amount of water vapor at a very low
  relative humidity than other materials. It is most effective where a
  low critical relative humidity is necessary, and were there is a high
  concentration of water vapor present.
  
  Calcium oxide removes water from a package very slowly, often taking
  days to reach its maximum capacity. As calcium oxide adsorbs moisture,
  it swells. Proper desiccant packaging is required for effective use.
  For these reasons, its use has been limited to primarily the packaging
  of dehydrated foods.


This following is a table which compares the properties of various dessicants taken from this site:-



And the following graph depicts the adsorption capacity of various dessicants:-

 

For more information click on the following sites:-


http://www.ohe-chem.co.jp/e_qa.html
http://www.agmcontainer.com/support/selecting-desiccant
https://www.usaemergencysupply.com/information-center/self-reliance/food-storage-frequently-asked-questions/types-of-desiccants