HildaC - I have a permanent colostomy, so my wardrobe options did change a bit, both because of stoma placement (left side, about four fingers below and to the left of my navel) and weight gain/loss from surgery, chemo, surgery, chemo, etc.
I had to maintain a business casual wardrobe - but the emphasis on truly casual when I wasn't leading a training class or running a videoconference or meeting with the bigwigs. I used to wear scrubs when I had to move equipment, so wore my scrubs to infusions (easy port access, comfortable pants with pockets.) And I wore jeans or leggings with longer tops, sweaters, and jackets depending on the season. My clothes, even pre-cancer, were never the body-hugging, midriff-baring, hip hugger type, 'cause I just didn't feel comfortable in that stuff even as a size "small." I definitely didn't feel comfortable in that kind of stuff as an XL after a year of continuous steroids.
But although I needed new sizes, I was able to stick to (mostly) the same types of clothes and same styles I'd been wearing before surgery. I did go to elastic waistbands, or partial elastic waistbands early on - and then found them so comfortable and the look professional enough that I really didn't seek out hard non-giving waistbands when I got to the point that I could wear them. Cancer has taught me that life is too short to be uncomfortable in my clothes!
I found some pleated front "poet style" longer shirts with adjustable long-to-short sleeves, and bought one in every color that I wear untucked (but sometimes belted) over pants.
For spring-summer-fall I love love love the boho elastic waist long skirts and embroidered jumpers and wear them with cotton t-shirts - during those seasons I can wear sandals and don't have to wear pantyhose. Things like basic tees, turtlenecks and shells all still fit me exactly as they did before - with an ileo, you might feel more comfortable sizing up, or you might want the tee or turtlenecks you wear under jackets and jumpers to fit you a little closer to help hold the bag stable against your body.
I definitely prefer separates to dresses, unless the dress is like the boho jumper - simple and easy to get in and out of.
Now, writing from home, I'm also wearing mostly danskin bra tops, yoga pants, hoodies and leggings around the house (with the occasional flannel shirt or sweater layered in on days like today when it's 14 degrees!) But I've got a few dress fleece and leather jackets, kept my business casual khakis, and add tee or turtleneck depending on the season so that I can look presentable for interviews and covering events.
BTW - you don't have to hit the mall. I built my post-ostomy wardrobe with the help of QVC, ebay and other online retailers. I'd experiment with the item sizing directly from QVC (who will always take it back) until I got the sizing right - and then go to eBay (NWT = new with tags) and buy the same item for a quarter of the price. And some things, like Denim & Co. jeans, leggings, leather jackets, fit so well I'll even pay the retail price. But online shopping can be a chemo patient's best friend. I haven't done face-to-face gift shopping for years!
Hope this helps.