Well one way, of course, would just to do a pressurized transfer (using C02) - we have FAQ's on how to do that and the gear you'd need (also in this FAQ section of the site)
Opening a valve on your fermenter means you will create back pressure in the system. To relieve that back pressure you are either going to suck in water/sanitizer from your blow off or bubbler or suck in some air from the outside world if you temporarily remove your bubbler or blow off.
If you are going to use gravity only, you'll need to remove the blow off tube or airlock temporarily. And yes, you will get a small amount of oxygen in there. The alternative is sucking in gross water/sanitizer from your blow off bucket or bubbler!
The amount of oxygen exposure here at this stage of your process really isn't a big deal. And if you are kegging or transferring to a secondary - whatever - you'll likely have more O2 exposure there than you would by just removing the bubbler or blow off tube for a moment when initiating your transfer.