Really good and thorough advice from Simon there, and I was going to suggest the spray poly trick as well.
I think that's what a lot people do when using Tru Oil with waterslide decals just to eliminate the chance of any smudging.

This is slightly off-topic, but hopefully someone finds it interesting...

First I'll point out that the name Tru-Oil is a little misleading. Whilst one of the ingredients is linseed oil, is not a pure timber oil. As Simon mentioned, is has a polymer in it (some form of polyurethane) and also a solvent component (likely a form of turpentine). Birchwood Casey are very secretive of their recipe.

On a side note, I have read numerous posts about making "homebrew" Tru Oil by mixing equal portions of the three ingredients "known" to be in Tru Oil (boiled linseed oil, polyurethane, mineral turps). I tried this thinking I could save a bundle of cash, but it just did not work well IMO.
It just did not have the the same build quality nor dry in the same time as the real deal stuff.
I tried varying the ratios and multiple coats and the results did not improve (this was over a few days time as I really thought I could crack it). The only thing I succeeded in was something that smelled just like Tru Oil!

In the end I suspect Birchwood Casey have some special drying agent in there because boiled linseed oil is notoriously slow to dry.
After all the experimenting, I decided either genuine Tru Oil or my DIY wipe on poly is the way to go for non-sprayed finish.