iOS - the iPad's operating system - has an 'Open In' facility. So, you download the movie in GoodReader then 'touch' the attachment and a pop-up box or menu will appear offering you the option to 'Open In' and a list of apps that can process that type of file. OPlayer will be in that list - maybe other apps too. If you select OPlayer, then iOS will make a copy of the file to the OPlayer file area and move you to the OPlayer app, from where you can play the movie...
Tim