Off spin to a batsman of the same hand (OS to RHB, SLA to LHB) is probably the most difficult to bowl in game because of the AI tendency to play just about anything on the stumps to leg side. For that reason I always try to position my fielders in positions that can prevent run scoring. I'll usually have a square leg, mid on, short mid wicket, then at least one player on the boundary (somewhere between square and deep mid wicket), depending on the circumstances or format. I'll sometimes have a leg gully to prevent them flicking behind. Depending on the length I'm bowling I'll have a deep backward point or deep cover/deep extra cover. A fourth slip feels like the best position for a slip fielder.
Depending on what control system you use I'd take advantage of the options to vary flight, bounce and length. I use standard controls for spin, I've found most success bowling full or short balls. I'm not sure if the adjusted length modifiers work for standard controls but I use them anyway - good length a little fuller usually. I think the Bounce, Flight, Standard deliveries also modify the length so I'd be conscious of that. I will adjust bowling position depending on the amount of spin. If there is rough try and find the length to bowl into that and adjust the field accordingly. It really does make a difference to the ball movement after pitching.
