I am trying to get Opera to re-request a page every time instead of just serving it from the cache. I'm sending the 'Cache-control: no-cache' and 'Pragma: no-cache' response headers but it seems as if Opera is just ignoring these headers. It works fine in other browsers - Chrome, IE, Firefox.
How do I stop Opera from caching pages? What I want to be able to do is have Opera re-request a page when the user clicks the Back button on the browser.
It sounds like your problem is related to this answer. After testing your header and the suggested headers, I could only reproduce your expected behavior in Internet Explorer.
As a user, I absolutely detest pages that slow down my history navigation by forcing re-loads when I use the back button. (If the browser you use on a daily basis paid attention to the various caching directives and let them affect history navigation the way you want as a developer you'd probably notice some sites slowing down yourself...)
If you have a very strong use case for doing this I'd say your architecture might be "wrong" in some sense - for example, if you're switching between different "views" of constantly updating data and thus want to enforce re-load when users go back perhaps using Ajaxy techniques for loading the constantly changing data into the current page would be better?
Opera's implementation is on purpose - "caching" is seen as conceptually different from "history navigation", the former is more about storing things on disk and between sessions, the latter is switching back to a temporarily hidden page you just visited, in the state you left it.
However, if you really, really need it there is a loophole in this policy that enables the behaviour you want. Sending "Cache-control: must-revalidate" will force Opera to re-load every page on navigation, but only if you're sending the page over https. (This is a feature requested by and intended for paranoid banks, it slows down way too many normal sites if applied on http).