I'd advise against closed O, unless you face IMPOSSIBLE odds. A Tight C with strong infantry or cavalry on flanks has the benefit of LURING enemy cavalry exactly where you want it.
If there is one gap in the wall of phalanx, this is where the enemy cav will go.
Group your archers (and skirmishers if you use them. I did) and put some strong units in the back line. When enemy cav charges your flank focus fire with your archers and charge with the melee. They will break like a twig.
You may have already seen this pic ( i posted it somewhere in Armenia LP )
This setup was enough to wipe out 3 full roman stacks at once.

Archers start outside, and after two volleys move behind the hoplites. Heavy Peltasts are decent for the role of flank infantry, and have javelins too.
Ignore the siege weapons and pack more infantry.
The two generals were going all Dick Mountainjoy on the enemy each fight. Taking out onagers, balistas, archers, slingers, skirmishers.. In that order.
As for players.. I advise against all phalanx armies. The terrible maneuverability makes them an easy prey.