Kuklinski's country was Free Poland (he was born into it, mind you), not Communist Poland. Communist Poland was an unlawfully and unconstitutionally created appendage of the Soviet takeover and occupation of Poland in the latter stages of, and after, WW2. Ergo, the "Oath" he purportedly took (which probably included a clause to the effect that he had to co-operate with the Soviets) was a construct of an unlawful regime, meaning it was not an Oath at all. But, for argument's sake, if it was an Oath, it would needs be broken to the extent that it conflicted with his over-arching obligation to Free Poland because, in law and in equity (if not 'in fact') Kuklinski's country was Free Poland, not Communist Poland.
To advocate otherwise is to turn the concepts of the rule of law and national sovereignty on their heads.
In any event, he was 'released' from his "Oath" upon the collapse of Communist Poland and Soviet Russia. Such release, in the circumstances, was retro-active by virtue of the fact that:
1. There was no-one (be it a person or entity) to whom the Oath could be upheld to as there was no-one to enforce it, nor was their any subrogation type arrangement.
2. Had the "Oath" remained binding (which presupposes there was subrogation from the Soviets, which there wasn't), the Post Communist, Free Polish Government (being the lawful and constitutional Government) would have prosecuted Kuklinski (or enforced it, but most likely issued punishment for breach), which it didn't.
Heartfelt and handwringing pleas to the concepts of honour and loyalty surrounding an Oath are just argumentum ad passiones.
It could be argued that Kuklinski served Poland whereas his fellow officers served a foreign power.
No need to argue it mate - the facts speak for themselves, no matter how hard some apologists try to dissimulate.
Given the reality of 'service' to the Soviets however (i.e. you either serve or you go to jail/get shot etc.), I don't think we can stand in judgment and condemn a very great many honest Poles who had no choice other than to survive under the Communist juggernaut (save and except those 'Poles' who relished their life under Communist rule and were part and parcel of the system).