I'd go with more or less the thought expressed by
Alamankarazieff and CapnZapp on this. It IS ambiguous and the DM will have to decide based on various factors on a case by case basis.
Factors which might be taken into consideration:
Does it require a sustain at all? If not then the case is pretty much clear, the caster isn't doing ANYTHING and any further attacks by the power are simply consequences of the original attack which OA has already been granted on.
Is it a sustain where the effect is again an ongoing effect in an area already targeted by the spell? This would be the case for any sort of 'zone' kind of effect where the enemy has a choice to move out of the zone or suffer damage, or the enemy simply suffers the damage because they are in the zone. Usually these effects also apply to allies if they happen to be in the zone.
What does the rules text of the power say exactly? If it describes the sustained effects as being an attack, then it begins to make sense to grant OA. I would still consider the nature of the attack. Is it restricted to the original target? Does it have all the attributes of the initial attack? If it is restricted to the original target then I would be inclined to rule it is more along the lines of 'further consequences of the initial attack' and doesn't warrant an OA.
The type of action required for the sustain would also factor in to some extent. A sustain standard that allowed independent attacks with the same or similar attributes as the initial attack I would be inclined to grant an OA against. In this case the wizard is obviously well occupied making the attack and it doesn't really seem logical to grant an OA on only the initial attack in this case since it really is no different from the ongoing attacks.
As a general rule of thumb I would go for 'OA minimization'. If it isn't fairly clear that an OA should be granted, then it probably should not be granted.
I think the case of the Flaming Sphere and its ilk are the thorniest. It seems that the wizard is essentially using the sphere as a weapon. It even says under sustain that making an attack with the sphere is a standard action, and I would grant OA when the wizard takes that action, but I would definitely NOT grant OA simply because a monster happened to be next to the sphere and took the 1d4 + INT damage. That IMHO is a consequence of the existence of the sphere, not an attack by the caster (and in this case it also lacks a to hit roll).
Bigby's Icy Grasp also has some similar language. Thus my ruling there would be as long as the wizard maintains the grab on the original target, no OA is incurred. If he uses the standard action option of sustain to attack a different target, then OA are triggered.
I know it isn't an absolutely clear rule that can be stated without equivocation in terms of specific rules text, but IMHO it does address the spirit of the OA rules and produces a balanced outcome. Actively attacking distant enemies results in OAs. I think expanding the OAs to ALL cases where a wizard happens to cause damage simply because a sustained effect exists and someone takes damage from it seems to me too much of a penalty for those kinds of powers use.