Remove OTA installed iOS 8 and reinstall on itunes
There is no difference in space required after the upgrade whether you upgrade iOS over the air or via iTunes. The only known problem is that you need more free space on your iOS device before/during the upgrade if you do it over the air. But as you already seem to have downloaded the upgrade to your phone this doesn't seem to be an issue for you, so just apply the upgrade on the phone and you are done.