Go Yishun for 851/811, Rare 171/853
Go Woodlands for Rare 925/926/969
Go Choa Chu Kang for 307/172/189 etc. Rare 190 etc.
OC500LEs are now very common for service 171 already. Not so
common ones are services like 854. Whereas service 853 do not
have any OC500LEs nowdays but rather Nissan UDs instead 
.
Other not so common OC500LEs in CCK includes service 67 and
188. In Bt Batok you can also catch OC500LEs on feeder service
945 as well.
For Woodlands you got to depend on luck on services like 169,
925, 926, 961, 965 and 969... 
