Just looked up Omnistor steps; they don't look much different than the Project 2000 steps found on later model Interstates. On the assumption that they're similar, I'll tell you about mine, and see if it rings a bell for you…
The Project 2000 step includes a "step lock" switch on the light switch panel by the sliding side door. The step lock locks the step in the "out" position, but not in the "in" position. However, it is possible to override the step lock by starting the engine. There's a relay, such that when the ignition key is turned on, it forces the step to retract.
My problem came in that the step lock switch on the wall is wired backwards from every other switch on that panel. All the other switches are off=down, on=up. The step lock is wired so that on=down, off=up. When I thought the step lock was off, it was actually on, hence the problem. Took a lot of experimenting to figure it out, too, because I never considered a problem with the switch.
If the step lock is on, and the door is opened (with the engine off), then the step will slide out as it's supposed to, and stay out even when you close the door. So far, so good.
The confusing thing is, if you open the side door while the engine is running and the step lock is switched on, and then shut off the engine while the side door is still open, the step will extend when you close the door, and the only way to make it retract is to start the engine again. Barked my shins on that darned step more than once before I figured out what was happening.
So, now I just always leave the step lock switched off except at a campsite where I won't be starting the engine again until I'm ready to leave.
Does this help you any in figuring out what's wrong with yours?
Engineering: Finding complex solutions to simple problems you didn't even know you had.