A pond aerator is a great choice for oxygenating your pond throughout the winter and keeping your fish healthy. Bubblers or agitators can also help mix the water and increase contact between the water and the air above. Ensure that you maintain a hole in your ice, keeping the pond from being completely sealed over. This will not only allow air to access the water, but will also allow gases to escape the pond.
Never create a hole in your ice by breaking the ice through force or smashing. This can send dangerous vibrations into the water that can hurt or even kill your fish. Instead, use a deicer or heater or boiling water to melt the ice and create a hole for oxygen to enter through and gas to pass out. Deicers are good if you're not going to be available to keep an eye on your pond for a few days or even a week. However, try not to run it too often, as this uses a whole lot of energy.
