Short answers, pulled from the story.
The Oder is 840 kilometers long in total, flowing through the Czech Republic, Poland, and along the border between Poland and Germany. It rises in the Czech Republic and flows 742 kilometers through western Poland before forming 187 kilometers of the Polish-German border, ultimately reaching the Baltic Sea.
The Oder-Neisse line is the border between Poland and Germany that runs along the Oder and Lusatian Neisse rivers. It was established by the victorious Allied powers at the Potsdam Conference after World War II. A reunified Germany and the Republic of Poland signed a treaty formally recognizing it as their permanent frontier in 1990.
In August 2022, the Oder was contaminated and at least 135 tonnes of dead fish washed up on its shores. Water samples taken on the 28th of July indicated possible mesitylene contamination, though the toxin was not present in samples taken after the 1st of August. The precise source of the contamination remained uncertain.
Several major medieval cities developed along the Oder, including Wrocław, which became the capital of Lower Silesia and one of the principal cities of the Kingdom of Poland; Opole, which became the capital of Upper Silesia; and Lubusz (now Lebus), which was known as 'the key to the Kingdom of Poland.' Wrocław's Catholic bishopric was founded in 1000 and Lubusz's in 1125.
Frederick the Great recommended diverting the Oder into a straight channel through the marshy area known as Oderbruch near Küstrin (now Kostrzyn nad Odrą). The work was carried out between 1746 and 1753, draining a large tract of marshland, cutting off a long meander, and confining the river to a new canal.
The Oder served as part of the Amber Road, the trade route that carried Baltic amber south to the Roman Empire. Roman writers recorded the river as the Viadrus or Viadua in Classical Latin. The Dagome iudex, dated to 990 AD, placed parts of the Oder along the western boundary of the Duchy of Poland under Duke Mieszko I.