I'd like to know if the wood filler applied to red oak strip
flooring to cover nicks and dents will be visible/ noticeable
If I do not plan to stain the floor. Ideally I would like to use OMU
And let it turn amber with time.

If its noticeable, is there a simple method to make it unnoticeable ?

Thanks

Some filled spots maybe more noticeable than others. Red oak flooring is quite varied in color and grain. The filler is often a single color so it may stand out more on very light boards. Also, OMU tends to make waterbased floor fillers look darker than if one uses an all waterbased finishing system. There is no simple solution. It's best to makes samples so you know what things will look like when you're done. If you're re-sanding the floors, it's best to sand out the dents. Or if a dent is very deep, consider replacing the board. To the trained eye, most filler is somewhat visible. The casual observer will probably not notice it.

Thanks, Gary.

I agree that red oak comes in different colors. Would it make sense to buy different wood filler colors to help blend them in better and make them less noticeable or lightly stain with a light color instead? What light stain look good with OMU on red oak ?

Some do that. The problem is, you don't know what the filler will end up looking like until after the floor is finished. Personally, I select a filler that best matches the overall color of the wood and use that.