Our fermenters have an oversized 4" port in the center which makes hop additions super easy. If you are using a cooling coil the port can still be used for hop additions. Simply unclamp the coil and slide it over which will give plenty of room for the hop additions.
You can purge any oxygen that made its way into the fermenter by doing a quick purge of carbon dioxide. You can do this by attaching the gas manifold to the blow off port, applying 5psi to the tank and then purging pressure from the pressure release valve.