this is exactly what my axie darnell is like, i took him to a vet and she drained him. you should try and get to a vet asap. how long did it take oscar to get like this?? is he still eating and pooing? what is he fed on?
the photo ive attached is after hed been drained, he was alot bigger but probably not as big as oscar, but it looks like it could be the same problem. Darnells gotta go back t vets again to be drained weds,
the vet said the fluid is either his kidneys failing or a nutritional deficiency, i hope oscar gets better please let us know
p.s where in uk are you i might be able to help you find a vet