As a junior at Lake Forest College in 1964, Harry Oppenheimer had a tent, a truck and a timely idea. He began with the Chicago debutante circuit, renting out his yellow-and-white-striped canvas for lawn parties, and the next winter followed society south to Palm Beach. Today Harry Darwin Oppenheimer II has more big tops than the circus. His company, HDO Productions, puts up at least 4,000 tents a year—111 of them in one recent week alone, from Beverly Hills, Calif. to Newport, R.I. The occasion may be an intimate dinner for 12 or the U.S. Grand Prix in Long Beach, Calif., where during three days last March HDO offered shelter to a crowd of 200,000, plus the Anheuser-Busch Clydesdales, with a 40-tent network.

Oppenheimer tents are found overhead at fancy showbiz parties (in the past for Frank Sinatra, Diana Ross, Barbra Streisand, Elton John and Rod Stewart). Harry's staff hammered stakes in Washington and Chicago during Pope John Paul II's visit, and provided the tangerine-and-yellow-striped tent in which Menachem Begin and Anwar Sadat celebrated the signing of their peace treaty at a dinner on the White House lawn last year. Quipped President Carter before the ceremony: "If they don't sign this thing, at least Amy and I will have a place [with enough room] to play Frisbee."

The 39-year-old proprietor of HDO has the well-bred air of a State Department protocol man, and indeed, says his business partner and college buddy, John Scallan, Harry has no peer for tact. "Some of our customers require understanding," Oppenheimer understates. "We are dealing with people at the most excitable moments of their lives." With 165 employees and offices in Chicago, Washington and Los Angeles, HDO also offers catering, decorations (from crystal chandeliers to balloons), music (plus a floor to dance on) and heaters for merrymaking on New Year's Eve. The cost varies from $350 for a 20-by-20-foot tent (which seats 40) to $20,000 for a 100-by-450-foot exhibition hall.

"Twenty-three years ago we were only an idea in the carport of my parents' home," Oppenheimer says. While in Highland Park (III.) High School, he started a parking service for parties in his affluent neighborhood. The idea of renting clean, bright party tents came to him when he looked around at those available and found them "old and grungy." He ordered splashy custom-made ones. In 1963 he and Scallan (who owns a percentage of the firm) formed HDO Productions. At 26, Harry bought out a prominent Long Island tent rental company and began doing business from Oyster Bay, Long Island to Philadelphia's Main Line. To look older and more dignified when dealing with clients, he wore dark-rimmed spectacles with plain glass for years.

Raising a tent is like a Broadway opening for Oppenheimer. "Harry and I are both inherent hams," Scallan confides. "I suppose parties bring it out." That once included acting nonchalant when nobody showed up because the hostess had forgotten to mail the invitations. After two hours of frantic telephone calls, the tent filled with come-as-you-are guests.

Oppenheimer's life has not always been so festive. His father, the owner of a sausage-casing company, died when he was 17, and his mother, Florine, has been crippled by polio most of her adult life. Now 67, she lives in one of two remodeled servants' cottages on 15 acres in Highland Park owned by her son. Harry, his wife, Buff, 35, and their two children reside in the other.

The tent king enjoys playing the role of country squire in his off-hours. He has seven riding horses in the stable and a swimming pool and tennis court on the grounds. The elegant hostesses he does business with would surely approve of the lush pastures, whitewashed fences and especially a sign by the entrance that warns, "Drive slowly. Dog sleeping in driveway."