Monster S4R
 

My bike is 5 years old and has done 13000 miles. Whenever I start the engine the oil pressure light stays on. I have changed oil and filter as they were due for replacement but the problem is still there - is there anything else I should check? I miss riding her!

Try replacing the oil pressure switch, if that cures it ok, otherwise it sounds like you will need to go to the dealer for them to put a proper guage on.

Its not uncommon for the Oil pressure switch to play up, as Chris says certainly worth trying another in its place
What area are you in as there maybe someone local who could lend you one