I have a mac mini and a macbook. I lost the OS X installation DVD for my macbook. would it be ok to use the disk I have for my Mac Mini?
It depends on the build number of Mac OS that is on that Installation DVD (printed on the DVD). If that number is higher than the OS version your Macbook shipped with, everything will work without problems, just make sure to install Mac OS X Software Updates afterwards!
Or in short: If the Mini is "younger" than your MacBook, it should work fine.