I happened upon an image of othismos as I describe it in my crowd-othismos model: men packed tight belly to back, not pushing side-on. This is an frame from the Discovery Channel's War and Civilization that can be seen on youtube.com. Here we can see the type of packing that must occur. Yes, I know the shields are terrible. You can see that in this environment the overhand grip on the spear will provide a much broader range of motion than even a high underhand grip. You'll also see that at this range the dory is useless for fighting in the front rank against your immediate foe. There is no way to choke up on the shaft far enough to bring an 8' spear to bear on the rank ahead of you. Thus, either the first rank used swords, or fought past the men in front, aiming deeper in the enemy ranks.
Another thing you can see is why I think the shields must overlap right over left, i.e: the man on the left comes up behind the overhanging shield of the man to his right. In the image below, the arrows show a weak point in the shield-wall when overlapped left over right as under the top arrow. The reason this joint gives way is that the portion of shield off to a man's left is easier to push back than the portion to the right. The flange to the left acts as a lever on the hoplites arm, while the right side, if forced back, is pushed into his body.
Now look at the lower arrow and you will see that pushing here only tightens the bond between shields and strengthens the wall, forcing both shields back into the body of the hoplite on the left.