Would it be a decisive advantage, if the opponent would not be allowed to castle? Suppose, you play a normal chess game, with the only exception that the opponent
 is not allowed to castle, but you are.

Do you have a decisive advantage ?
In fact, Larry Kaufman (Komodo) did computer testing on this with Komodo, and found that White has about 50% winning chances when Black starts with no castling privileges. So it's a toss-up.

http://talkchess.com/forum/viewtopic.php?p=650339#650339


  Here is a perfect example: In the initial position, forbid Black from
  castling. Komodo gives that position a score that is around where I
  think the dividing line between win and draw lies. I had Komodo 9.3
  play it out on four threads (to insure variety due to MP
  unpredictability) a hundred times at game in one minute. getting 49
  White wins, 46 draws, 5 Black wins. Then at two minutes plus one
  second, a similar result: 49 White wins, 49 draws, 2 Black wins. So
  just forbidding castling for Black appears to put the game right on
  the win/draw line. Of course results could be different at TCEC time
  controls on many cores, but it's not obvious whether the White win
  percentage would go up or down.


There have been some who have dubbed this "Kaufman chess", and suggested it as a sort of long-time control Armageddon method (rather than two rapid games, then two faster games, then two blitz games, and finally a blitz Armageddon, as in the FIDE World Cup).