OK - I'm not a vet either, but I can tell you with all my experience in nursing....insulin will not cause oliguria. But I do know that oliguria and anuria are medical emergencies that require immediate treatment. If left untreated, these conditions can lead to death within a very short period of time as his kidneys will fail.
Various conditions may be responsible for different types of oliguria and anuria. I'm not going to get into the particulars right now....as it's more important for the moment to figure out what's going on.
I have to ask you, what specifically did your vet do today in response to no output. I'm going to assume since you report no obstruction that some of the key diagnostic tests were done to help rule out an obstruction, i.e. urethrocystoscopy, (which uses an insertable diagnostic tool to view the interior of the urinary tract and bladder wall, a urine analysis, an electrocardiograph , abdominal radiographs or ultrasounds.
Please confirm this and also let me know if any blood work was done, and if yes, any abnormal results. Did they catheterize Heathy for a urine specimen and if yes, any abnormal results?
Lastly, has Heathy been drinking less lately? Had vomiting or diarrhea? Does he appear to be "swollen" anywhere on his stomach? If you look at his mucous membranes (inside his lips would be easiest), does it seem dry and pale or pink and moist?
I'll wait for your response.
I'll wait to hear from you.