I'm sorry to hear your Big O fish is having a problem. While there is no way to be certain based only on an online description, the feeding behavior you have described is often a sign of something being caught in the mouth or throat. Because fish breathe through their gills, they don't choke as a mammal might. A fish with this problem may also sometimes swim head down or try to shake its head to dislodge the object. It could be a rock, a piece of stick that has fallen into the pond, or any of a number of foreign objects. Sometimes a fish will finally manage to get the object out, and many pond keepers do nothing. However, a fish can become greatly distressed from this and eventually die of exhaustion. To help, you'd need to catch the fish. Wrap it in a wet towel, and hold it head down. Rub the throat in one direction - toward the mouth- to see if that will dislodge the object. If that doesn't work, you can open the fish's mouth, and using round-ended tweezers very gently take hold of the object and dislodge it.
If you don't see anything, there could be something further into the esophagus. In that case, there won't be anything you can do on your own. You can wait to see if the koi will be able to get rid of it, or an aquatic vet could remove it. A polyp or tumor in the throat or esophagus could also cause this symptom. You would need a vet to take care of that, too. If you need a vet, these links will take you to directories of fish vets:
If you have more questions, let me know by clicking on REPLY. I hope Big O will be fine.
Please remember to rate my service once you have all the information you need. If you have any other questions, please ask me – I’ll be happy to respond. Thank you!