I've been plagued by this myself... here are my pointers.
With the Rotex 125 it is best to try and keep the unit level at all times and find a comfortable place for your hands so the machine stays stable. It is easy to apply uneven pressure which if in the fine orbiting mode can cause swirls from the pad not staying at a constant speed. That said, some woods play nicer with the sander than others. But in general let the sander do the work. I normally keep one hand on top and one hand along the front to help guide it. But I try and not apply too much physical pressure downward. The ergonomics of the Rotex are such that with the barrel diameter being quite large there is an issue of fatigue as in cases of all day sanding etc.
Another issue is the mount for the pad itself. If it is a well used and worn mount then swap it out for a new one. I mostly use the rigid mount. But in general you want to make sure the mount is flat and can spin without interference. This is the same for soft of cushioned mounts, since, these have a tendency to set the machine off balance and hence the speed of sanding will vary.
If you do find swirls between grits then step back to the previous grit before continuing. I often will do one light hand sand of the surface before moving on if I think there might be an issue. Denatured alcohol helps with seeing the swirls before applying oil. I also will do two passes the length of the piece of wood and two passes back and forth across the width. Because the nature of random orbital is to leave finer and finer swirls, meaning, the machine orbits in a circular pattern which means there are always "swirls" but they just aren't visible, so, it helps to go twice in the direction of the grain and twice across the grain to aid in the "blending" of the marks etc.
A perfectly flat surface will always give finer results than a slightly uneven surface.
Finally, if you are worried about even the smallest swirls or "pig tails" showing up during oiling, then, I have found going to 220 for large surfaces to be sufficient. I start at 120 if the material is planed and jump to 220 directly. For smaller surfaces I sometimes go higher. But going higher will increase the possibility of seeing swirls. I also found that in some cases after oiling you can use the sheep skin buffer or something similar to help knock down the sheen on a difficult surface. This also makes it harder to see any minor swirls. But that always bugs me knowing they are there!
Good luck!