What OS are you running? How much RAM do you have?
This is from the
Opera Users FAQ:
For those running out of resources on win9x systems, try this:
1. Go to preferences->History and cache.
2. Uncheck / disable automatic RAM cache
3. Set both Cache documents and Cache images to 10000 KB and make sure they're checked.
If you still run into problems, try lowering the "Cache images" value to 8000, then 5000 if problem persists.
I run Win200/XP and manually set my cache at 10megs each. The most RAM usage I've seem in TaskMan is around 25MB and that's with LOTS of open Opera windows.