Answer by taking a quote from the following article:

Oscar Palmer Robertson (born November 24, 1938), nicknamed "The Big O", is an American former National Basketball Association player who played for the Cincinnati Royals and Milwaukee Bucks. The 6 ft 5 in (1.96 m), 205 lb (93 kg) Robertson played point guard and was a 12-time All-Star, 11-time member of the All-NBA Team, and one-time winner of the MVP award in 14 professional seasons. In 1962, he became the first player in NBA history to average a triple-double for a season. In the 1970-71 NBA season, he was a key player on the team that brought the Bucks their only NBA title.

Prior to the 1970-71 season, the Royals stunned the basketball world by trading Robertson to the Bucks for Flynn Robinson and Charlie Paulk. No reasons were officially given, but many pundits suspected head coach Bob Cousy was jealous of all the attention Robertson was getting. Robertson himself said: "I think he was wrong and I will never forget it." The relationship between Oscar and the Royals had soured to the point that Cincinnati had also approached the Lakers and Knicks about deals involving their star player (the Knicks players who were discussed in those scenarios are unknown, but Los Angeles stated publicly that the Royals asked about Jerry West and Wilt Chamberlain, with the Lakers saying they would not consider trading either star).  However, the trade proved highly beneficial for Robertson. After being stuck with an under-performing team the last six years, he now was paired with the young Lew Alcindor, who would years later become the all-time NBA scoring leader as Kareem Abdul-Jabbar. With Alcindor in the low post and Robertson running the backcourt, the Bucks charged to a league-best 66-16 record, including a then-record 20-game win streak, a dominating 12-2 record in the playoffs, and crowned their season with the NBA title by sweeping the Baltimore Bullets 4-0 in the 1971 NBA Finals. For the first time in his career, Robertson had won an NBA championship.  From a historical perspective, however, Robertson's most important contribution was made not on a basketball court, but rather in a court of law. It was the year of the landmark Robertson v. National Basketball Ass'n, an antitrust suit filed by the NBA's Players Association against the league. As Robertson was the president of the Players Association, the case bore his name. In this suit, the proposed merger between the NBA and American Basketball Association was delayed until 1976, and the college draft as well as the free agency clauses were reformed. Robertson himself stated that the main reason was that clubs basically owned their players: players were forbidden to talk to other clubs once their contract was up, because free agency did not exist back then. Six years after the suit was filed, the NBA finally reached a settlement, the ABA-NBA merger took place, and the Oscar Robertson suit encouraged signing of more free agents and eventually led to higher salaries for all players.  On the hardwood, the veteran Robertson still proved he was a valuable player. Paired with Abdul-Jabbar, two more division titles with the Bucks followed in the 1971-72 and 1972-73 season. In Robertson's last season, he helped lead Milwaukee to a league-best 59-23 record and helped them to reach the 1974 NBA Finals. There, Robertson had the chance to end his stellar career with a second ring. The Bucks were matched up against a Boston Celtics team powered by an inspired Dave Cowens, and the Bucks lost in seven games. As a testament to Robertson's importance to the Bucks, in the season following his retirement the Bucks fell to last place in their division with a 38-44 record in spite of the continued presence of Abdul-Jabbar.  Robertson was elected to the Wisconsin Athletic Hall of Fame in 1995.

what did Robertson accomplish during his time with the Bucks?
the Bucks charged to a league-best 66-16 record, including a then-record 20-game win streak,