In my experience, almost anyone can be an offspin bowler, but it's very hard to be a good offspin bowler.
Here's what I think makes a good off spinner hard to face
1. accuracy
2. Subtle changes of pace
3. Changing position on the crease
4. Drift
5. Bowling to a plan
Also, don't be afraid to get hit for six
I think this is kinda true. Its easy to be a bad spinner. Its hard to be a good spinner.
Offspin and legspin are equivalently hard, the difference is that a bad legspinner is bad in more obvious ways than a bad offspinner. Bad offspinners tend to just bowl the same vaguely unthreatening ball over and over. Bad legspinners are all over the place.
To be a good spinner you need:
a) to be able to get good revs on the ball
b) to be able to land it on a line and length 5 ball out of 6
c) to be able to bowl at a decent enough pace that batsmen can't wander down the wicket and hit everything on the full toss, or camp on the back foot and play you off the pitch
d) to have at least 1, preferably 2, decent variations - either change the direction of spin or mix in some topspin or backspin.
e) to develop a reasonable understanding of tactics - setting the field, reading the batsman, making the necessary adjustments