There are many factors that will effect the oxygen level in your pond but the main ones are:
Temperature: Warm water has a lower ability to hold onto oxygen than colder water
Animals: The more pond life you have in your pond the higher the amount of oxygen that is required and used up
Water Clarity: Pond plants need light to generate oxygen. The clearer the water the more effective your plants will be
To improve water clarity you may want to consider a pond additive. An air pump will assist in increasing oxygenation in your pond.