Karol, I am no expert, but since no one has responded yet, I will try to help. First, are the fishies gasping at the surface of the pond?
Are you sure there isn't another cause? Could also be parasitic, toxins leaching from surrounding yard, have any chemicals been used near the pond? even is sprayed from a ways off, the air can carry it to the pond
If you think it is oxygen, you can disturb the surface of the pond to get more oxygen. an aerator will do that with an airstone. You can get a small one from an aquarium shop (buy the biggest one they have for a fish tank) or if you have a pond store near you, you can pick one up for a pond along with the tubing and the air stone. You can get help at either store for waht you need. You can also put a hose on a spray nozzle and let it spray the surface. Of course you will have to watch for chlorine and the pond level, but it's a short fix. Hope some of this helps. I'm sure others will chime in and have other ideas for you!