I am using an oak effect Danish oil and have applied it to a couple of test pieces of pine, one with 3 coats with 30 mins between coats and one with a single coat, and left overnight.
The single coat looks lighter and better to me. All advice I read says to apply 3-4 coats, or until the wood stops absorbing. Would it be okay to leave as a single coat?
This is tricky as a coloured version of a finish is doing two jobs at the same time, providing colour wood and adding protection.
For many finishes three coats is considered the minimum number to provide good protection. But when it comes to colouring the wood you should of course stop when you've gotten the colour you want. Normally that's not a problem as you then add protection on top of the colouring agent (often dye or stain) in the form of a clear finish.
Now how tough the surface of wood needs to be varies a great deal. Many things don't need finish at all, others do need something or they'll get stained or marked from use (scratches, dings, staining from water and other liquids).
So depending on what the application here is you can safely stop after just the one coat, but if the item needs more than minimal protection it would be best to apply further finish. The two leading options are a regular version of "Danish oil" or a varnish.
Either of the above will slightly alter the colour of the wood but it shouldn't be enough to be a problem.
