Hello Stue, have you checked with the Opera FAQ's regarding cache options? I never really looked into this back when I was running v.5, but with v.6,I do know that ALT+P/History and Cache will give you all the cache options you could possibly want. Here is a good source for many Opera insights and tips: [searchengineworld.com...]
Someone may come along who has direct experience with Opera 5 cache options, but do take a look at the info available through the link. Though it relates directly to Opera 6 there are many references to v.5 behaviours throughout.