Cristoforos,
I do not have any resources per say, other then my last name is about 32 letters long. 30 of which are constanants and the other two vowels, and then it ends in a "ski." I have many relatives that are from Poland, and many still in Poland. I have been over there for several family type occasions. I've actually been to a Orthodox church in Warsaw and one in Bialystock. I can tell you the some of the stuff I have seen or heard about first hand.
If your researching the 20th century, you'd probably do best to break this into 2 timeframes. Pre-Solidarity and Post-solidarity. (possibly even a third now). The type of persecution and the actual persecuter would chnage. In pre-solidaritytimes, the polish gov't was really a front of the Soviet Union. In Poland, like in all Soviet countries of the time, the gov't would persecute and systematically try to close churches of all types. They tried to close my Grandfathers church, but thats a different story. Though compared to other soviet block countries, Poland did enjoy a certain level of religious liberty. They didn't suffer near like the Christians in Russia or elsewhere. There should be plenty of resources available for persecutions in this time frame.
In post-solidarity Poland. The gov't really takes a back seat and a new persecuter of Orthodox Christians comes too. (I know by your faith icon, your not going to want to hear this.) But it was the Catholics, and at times the Catholic church. It wasn't persecution along the lines of closing churches or martydom. It was much more subtle. It was really discrimination, not persecution. After the rise of solidarity and the fall of the Soviet regime, Polish people became real suspicious of anything Russian. It was stuff like; they wouldn't hire you if you we're Russian. They wouldn't do business with you if you we're Russian. (and in a Polish persons eyes at the time: Orthodox = Russian. You could be as polish as a kielbasa, but if they heard you we're Orthodox. That meant you we're Russian.) Whenever I talked to my Grandparents or others in their age bracket. You can hear their bias. It really was a distrust of anything Russian becuase they just came out of a soviet regime, but the Orthodox church got tangled up in the stereotypes. An equivalent in America, would be how we treated the Irish immigrants up in the Northeast; particulary in Boston. This period is probably largely undocumented.
They are now probably entering a third phase. The older generation, with their distrust, are passing away, and the new generations do not have the same preconceived notions. So those days are really gone. Today their environment is pretty just like here in the states.
In addition. Geography would also have played a role. In the far west, the countries borders have moved many times. So in places like Bialystock, there are larger populations of Russians. (I typed that last sentence without really thinking, I was going to change it, but I think it shows a point. You can see I used the stereotype I was talking about above. Its somewhat engrained. The Bialystock people are obviously Polish, but yet I subconciously called them Russian.) Bialystock has a beautiful museum of old Orthodox Icons. No trip to Poland is complete without going there. Awesome!
Sorry no resources off hand. Just first hand insight.