I use a bladder tank in between my ro and my di. No oscillations. When it turns on, it takes a while to build up pressure to turn off again. And when you want water you get a good flow for at least a quart or two.

Sorry, my internet sucks right now. I spent half an hour searching for a link. But it's a blue tank you see in some pictures of RO/DI systems. I've seen them at Home Depot.

Available at Home Depot I think.

Jack