Is there an optimum OBS in AirlineSim?
Yes. How you define that optimum, that is up to you
This game is about you deciding your concept and making it work! Just like in real life, every decision and choice you make has its advantage and its disadvantage. There is no secret formula and no special method. You have to just toy around with it until you find the optimum rating for the minimal cost. For instance, I have found that you don't hit the 5 bar/star rating with business class until you spend a minimum of $40-ish per passenger! it is about balance. It is the same thing with other components such as plane choice and maintenance. You have to make a management decision based on your competition and reality. Good luck.
And generally, passengers seem to love eating twice! Bleeding gluttons!! :D :D :D :P
.... and they like to have lot of drinks, good seats, lots of flight attendants and new jets... :)