The Isreali-Palestinian conflict began in the 20th century. It's over who gets what land and how the land will be run. Jews wanted a national homelands but Arabs insist the land was there's to begin with.
The issues that make the whole situation complex is then fact that neither want to go along with the "one state" or "two state" solution.
The borders have changed because in the two solutions the land gets separated or divided. It can either become all one big Israel or one big Palestine. They could also name Palestine it's own independent state leaving the rest of Israel
The Oslo Accord was a set of agreements between thr government of Israel and the Palestine liberation Organization. The so called " peace treaty" was successful because each side agreed to the others negotiations.