Hello, my name is***** and I have over 20 years of experience as a veterinarian.
I am sorry to hear about Sully's chronic vomiting. Hairballs don't cause as much vomiting as you've seen with your fellow. Even with long haired cats we rarely see vomiting from hairballs more than once or twice a month. Sometimes excess grooming can be a way to self comfort when a cat doesn't feel well, which will lead to more hair in the vomitus because hair will cause irritation and vomiting.
As well olive oil is a digestible oil, so it isn't very effective for hairballs. I recommend the commercially available products like Petromalt or Cat Lax, which have petroleum based, non-digestible lubricants.
Chronic vomiting can be related to chronic pancreatitis, esophageal reflux, a dietary allergy or sensitivity, inflammatory bowel disease, heartworm
disease, internal organ failure, hyperthyroidism
, a full or partial gastrointestinal obstruction or even infiltrative cancers such as lymphoma.
Ideally he would have some testing done. I would check a complete blood count, biochemistry profile and T-4. If those aren't diagnostic then I recommend a heartworm test, as vomiting is strangely one of the most common signs of heartworm disease in cats. I would also check a specific blood test for pancreatitis, a fel spec PL (feline specific pancreatic lipase). This test is highly specific for pancreatitis in cats.
If those tests have been done or are done and come back normal the next step is an abdominal ultrasound and/or either exploratory surgery or endoscopy to gather biopsies of his gastrointestinal tract to look for inflammatory bowel disease or cancer.
In the meantime you can try giving him an acid reducer to see if that helps. You can try either:
1) Pepcid ac (famotodine) at a dose of 1/4 of a 10mg tablet per 5 to 10 pound cat every 12 to 24 hours
2) Prilosec (omeprazole) at a dose of 1/4 of a 10mg tablet per 5 to 10 pound cat every 24 hours.
These are quite safe and can be used long term as needed.
You could also try feeding a pet store canned only sensitive stomach diet such as those made by Royal Canin or Hills. If those aren't restrictive enough then prescription foods like Hills i/d or Purina Veterinary Diets EN or perhaps a hypoallergenic diet such as Hills z/d or Royal Canin Duck and Green Pea can be tried.
Please let me know if you have any further questions.