The Manischewitz is Kosher. The O-U stands for the Orthodox Union, Other common symbols you might see are A K in a circle O-K , A K in a star The Star K and the hebrew letter Kof which looks like a backward C with the K in the middle (A Kof-K)
About the Dunkin Donuts, There is a idea of “Jewish Milk” or Chalov Yisroel, It is milk that was milked by a Jew , bottled by a jew etc. It used to be before Pasterazation things like Pigs Milk were used to keep milk fresh (Pigs Milk isnt kosher) About 50-60 years ago (I am not sure when exactly) a major Rabbi in the US Rabbi Moshe Feinstein rules that in the US Milk is from a cow and cannot be from a Pig because the government enforced the laws about Milk and most milking is done by machines anyway that only fit a cows udder and not a Pigs udder.
Many more Charedi groups did not accept this and stick to the Kosher Milk only.
IMO and most people who deal with Kiruv issues, They would not recommend you take up this stringency unless you are ready or want to. It is perfect OK and Kosher to Eat Breyers or Eddys Ice Cream and the Milk you buy in the supermarket.
The Dunkin Donuts thread was about the Kosher Milk, It is likely that the Dunkin Donuts nearest to you is NOT Kosher though, most are not, only a few in NYC and Baltimore areas are Kosher to my knowledge