Vilas, Jimmyg and the others, thank you for your valuable inputs.
I was aware of the CI O as equivalent to best ROC but it looks like the reality with the ATC restriction and the wind always changing, an heavy A321 with a high IAS of around 280/290kt often gives you a very poor sometimes zero ROC for few seconds as the aircraft is pitching down to keep the speed. Flying in Hong Kong airspace every day, we have to reach cruising altitude before exiting their airspace. Sometimes, especially in winter with very strong tail wind increasing as we climb, even flying at Green dot speed the whole way we can't make it. But with a fairly light A321 or A320, I like using vertical speed for the last 5000 feet or so as you can aim to reach your cruising altitude exactly at the waypoint required by ATC. No need to workout what is the Best IAS, just set the required vertical speed with a safety margin as you might need to reduce the ROC at higher altitude. Works pretty well. On A319 I always fly managed speed unless restriction from ATC as its climb performance is never a concerned.