The most important incident in the Old Testament, and one on which most of the balance is based, is the story of God's covenant with Abraham preserved in Genesis 22:

The angel of the Lord called to Abraham from heaven a second time and said, “I swear by myself, declares the Lord, that because you have done this and have not withheld your son, your only son, I will surely bless you and make your descendants as numerous as the stars in the sky and as the sand on the seashore. Your descendants will take possession of the cities of their enemies, and through your offspring all nations on earth will be blessed, because you have obeyed me.

God elaborated further on the Covenant and how it was signified in Genesis 17:

As for me, behold, my covenant is with thee, and thou shalt be a father of many nations. Neither shall thy name any more be called Abram, but thy name shall be Abraham; for a father of many nations have I made thee. And I will make thee exceeding fruitful, and I will make nations of thee, and kings shall come out of thee. And I will establish my covenant between me and thee and thy seed after thee in their generations for an everlasting covenant, to be a God unto thee, and to thy seed after thee. And I will give unto thee, and to thy seed after thee, the land wherein thou art a stranger, all the land of Canaan, for an everlasting possession; and I will be their God. And God said unto Abraham, Thou shalt keep my covenant therefore, thou, and thy seed after thee in their generations. This is my covenant, which ye shall keep, between me and you and thy seed after thee; Every man child among you shall be circumcised.

This covenant marked the descendants of Abraham as God's chosen people. Virtually every passage of the Old Testament thereafter relates to this promise: the exodus of the people of Israel from Egypt to the land God promised; the capture of the Promised Land by Joshua; the punishment of the people when they abandoned the covenant, and the establishment of the Davidic line from which Jesus was descended. Even today, Jewish people believe that Israel is the land promised to them by Abraham.   

There are many significant events in the Old Testament. It opens with the God's creation of the world:

1: In the beginning God created the heaven and the earth.
2: And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters.
3: And God said, Let there be light: and there was light.

After God creates the world, he then creates the first man, Adam, and the first woman, Eve.

The next important event is Adam and Eve eating the fruit of the tree of knowledge and being expelled from Eden.

The murder of Abel, son of Adam and Eve, by his brother Cain, is, according to the Old Testament, the first fratricide, and in fact, first murder, in the world.

Many other important events revolve around the theme of God punishing humans for sin but saving small numbers of the righteous, e.g. the story of Noah and the Flood or the destruction of Sodom and Gomorrah.

The story of the captivity of the Jews in and their exodus from Egypt is also important for the history of Judaism and also Christianity (especially the Passover story).