Hello, my name is Dr. Kara and I have over 20 years of experience as a veterinarian.
I am sorry to hear that Oscar is experiencing "stress cystitis" which I believe is the same as what we call interstitial cystitis.
Interstitial cystitis is a poorly understood inflammatory condition that causes thickening of the bladder wall, discomfort, a poor mucous layer on the bladder wall and bleeding of the bladder lining. It seems to worsen with stressful conditions.
Treatment options are varied although none seem to help all cats. I recommend medication to relieve stress such as amitriptylline or buspirone and feliway (a synthetic calming pheromone), an omega 3 fatty acid as well as a glucosamine supplement as some studies show those decrease inflammation and improve the health of the mucous layer lining the bladder. Some veterinarians try steroids as well.
I also recommend increasing fluid intake as much as possible via canned diets, a kitty drinking fountain and offering low salt chicken broth or tuna juice to drink. See this link for a very good article from a respected veterinary researcher on interstitial cystitis: http://secure.aahanet.org/eweb/images/AAHAnet/phoenix2009proceedings/pdfs/01_scientific/103_UPDATES%20ON%20IDIOPATHIC_I.pdf
In some cats increasing play and positive interactions with housemates and their owners also seems to help. Playing more with Oscar, especially with interactive toys like fishing line things he can chase, laser pointers and toys he can retrieve seem to be favorites.
If he is an indoor only fellow sometimes getting him safely outdoors helps. Most cats aren't thrilled with a leash but some owners build outdoor cat pens so that they can safely be outdoors. These can be built even on a balcony if you live in an apartment.
See this link for some examples:
Best of luck with your fellow, please let me know if you have any questions.