I think you may need to show us the whole picture instead of the this iRule. The syntax is good, the logic is good. And I played your iRule here,
curl -b "OracleCookie=OraB" http://192.168.28.251/
curl -b "OracleCookie=OraC" http://192.168.28.251/
curl -b "OracleCookie=OraD" http://192.168.28.251/
Mar 18 18:30:41 local/tmm info tmm[6224]: 01220002:6: Rule Devcentral_1 : INTB-Pool-1
Mar 18 18:30:47 local/tmm info tmm[6224]: 01220002:6: Rule Devcentral_1 : Pool-2
Mar 18 18:30:54 local/tmm info tmm[6224]: 01220002:6: Rule Devcentral_1 : Pool-3
It's working for me. Do you have the sample of your cookie string? Since mine is quit simple.