All of the HP OB expressions are matured 100% in sherry casks. However, they DO make use of a combination of American oak and Spanish oak casks. The 15 year uses more American oak in the mix than the 12 or 18. They also vary the percentage of first fill casks between expressions.
The only specifics I could find on the 15 year are that it is matured in 30% first fill casks, vs 20% for the 12 year, and that there is a "higher proportion" of American oak casks.
I put the info I was able to find into this blog post
a while back.