Answer by taking a quote from the following article:

Oklahoma! is the first musical written by the team of composer Richard Rodgers and librettist Oscar Hammerstein II. The musical is based on Lynn Riggs' 1931 play, Green Grow the Lilacs. Set in Oklahoma Territory outside the town of Claremore in 1906, it tells the story of farm girl Laurey Williams and her courtship by two rival suitors, cowboy Curly McLain and the sinister and frightening farmhand Jud Fry. A secondary romance concerns cowboy Will Parker and his flirtatious fiancee, Ado Annie.

At the social, during an upbeat square dance ("The Farmer and the Cowman"), the rivalry between the local farmers and cowboys over fences and water rights has led to fighting, which Aunt Eller ends by firing a gun to silence everyone. Laurey is upset when she sees Curly at the dance with Gertie. In an effort to rid himself of Ado Annie, Ali Hakim buys Will's souvenirs from Kansas City for $50. Jud also contributes to this by purchasing Will's Little Wonder, knowing of the blade concealed within it. The auction starts and Will bids $50 on Ado Annie's basket, not realizing that without the $50, he would no longer have the money her father insisted he needs to "purchase" marriage with her. Desperate to be rid of Ado Annie, the peddler bids $51 to get the basket so that Will can approach Andrew Carnes with the $50 and claim Ado Annie as his bride. The auction becomes much more serious when Laurey's basket comes up for auction. Jud has saved all his money so he can win Laurey's basket. Various men bid, trying to protect Laurey, but Jud outbids them all. Curly and Jud engage in a ferocious bidding war, and Curly sells his saddle, his horse, and even his gun to raise money. Curly outbids Jud and wins the basket. Jud discreetly tries to kill Curly with the Little Wonder, but his plan is foiled when Aunt Eller (knowing what is happening) loudly asks Curly for a dance. Later that night, Will and Annie work out their differences, as she reluctantly agrees not to flirt with other men ("All Er Nuthin'").  Jud confronts Laurey about his feelings for her. When she admits that she does not return them, he threatens her. She then fires him as her farm hand, screaming at him to get off her property. Jud furiously threatens Laurey before he departs; Laurey bursts into tears and calls for Curly. She tells him that she has fired Jud and is frightened by what Jud might do now. Curly, seeing that she has turned to him for guidance and safety, reassures her and proposes to her, and she accepts ("People Will Say We're In Love (Reprise)"). He then realizes that he must now become a farmer. Afterwards, Ali Hakim decides to leave the territory and bids Ado Annie goodbye after telling her Will is the man she should marry.  Three weeks later, Laurey and Curly are married and everyone rejoices in celebration of the territory's impending statehood ("Oklahoma!"). During the celebration, Ali Hakim returns with his new wife, Gertie, whom he unwillingly married after being threatened by her father with a shotgun. A drunken Jud reappears, harasses Laurey by kissing her and punches Curly, and they begin to fist fight. Jud attacks Curly with a knife and Curly dodges, causing Jud to fall on his own knife. Jud soon dies. The wedding guests hold a makeshift trial for Curly, at Aunt Eller's urging, as the couple is due to leave for their honeymoon. The judge, Andrew Carnes, declares the verdict: "not guilty!" Curly and Laurey depart on their honeymoon in the surrey with the fringe on top ("Finale Ultimo").

What is the main event in act 2?
The auction becomes much more serious when Laurey's basket comes up for auction.