Goldfish and koi have lived in outdoor ponds for centuries, surviving all seasons. However, you cannot trust just any fish to survive a harsh winter. Additionally, even hardy pond fish like goldfish or koi need certain preparations in order to ensure that they will survive the freezing cold.
Fish have adapted to the water parameters -- including temperature -- of their native range. Many fish get very stressed when subjected to temperatures outside of their ideal parameters. For this reason, many common aquarium fish cannot survive in outdoor ponds. However, both goldfish and koi originate in sub-tropical and temperate conditions, like those in much of most of the United States and Europe. These fish can survive in outdoor ponds with proper precautions.
When winter is coming, you need to take some steps to ensure the health of your pond. If you have any tropical fish, you should start to move them inside when the water temperature drops below 70 degrees Fahrenheit. For goldfish and koi, stop feeding your fish when the temperature hits 50 degrees Fahrenheit. Additionally, only keep goldfish and koi outdoors in ponds greater than one foot deep, preferably with a deep spot at least a foot deeper than the rest of the pond.
Fish overwintering in a pond have a few things going for them. First, cold water holds more oxygen than warmer water, and cold goldfish, koi and pond bacteria use less oxygen. However, they will need some oxygenation over the winter. You can ensure you fish get enough oxygen by using a de-icer, available at stores that sell pond equipment or online. These melt a hole in the ice through the winter. You can also wait until the surface of the water freezes solid, then lower the pond water an inch. This creates a layer for gas exchange while the air provides additional insulation, preventing the water from getting too cold.
Live pond plants can cause problems in freezing conditions. Dead plants can rot and fester, even in winter temperatures. In order to prepare for winter, remove any tropical species. Most outdoor pond plants from the tropics can survive a winter in a garage, wrapped in damp newspaper -- so long as the temperature stays around 50 degrees. For temperate species, trim them down to six inches from the base, roots or tubers and remove the trimmings. In this state, most temperate plants can survive the winter alongside your goldfish or koi.