During his 2006 race for Attorney General of Texas, he vowed to fight for the people of Texas against the large monopolistic oil, insurance and media conglomerates, saying he'd "Fight them until hell freezes over and then fight them on the ice." Much of his funding for his 2006 campaign came from contributions of $20 to $100 from low to medium income donors who met him during his 254 county "Whistle stop" Campaign Tour or read about him on the blogs. His opponent, Republican incumbent Greg Abbott entered the race with over six million dollars in his campaign war chest.
As the Democratic nominee for attorney general, Van Os took his campaign directly to the people. He pledged to visit all 254 counties in Texas during the 2006 election cycle. With his wife Rachel Barrios-Van Os, who served as his campaign manager, he visited each courthouse and spoke on the courthouse square. His last five "Whistle Stops" were in Tarrant County on October 16, Dallas County on October 17, Harris County on October 18, Travis County on October 19 and his home county of Bexar on October 20. Local supporters joined him on the courthouse steps as he proclaimed his determination to "return Texas Government back to the people" denouncing the "bought and paid for perversion of the American political process." In Tarrant, truckdrivers passing on the street, acknowledged him by honking their horns when they saw the signs with his campaign message "MESSAGE TO BIG OIL -- I'm coming after you." During much of 2006 gasoline prices had soared to a record of over three dollars a gallon and dipped rapidly a month before the general election by at least a dollar a gallon. Many people suspected that the timing of the rise and fall was due to political manipulation. Supporters of David Van Os funded billboards in many counties in Texas with the following message: "MESSAGE TO BIG OIL - I'M COMING AFTER YOU!" His opponent, Abbott, received considerable money from insurance companies. Van Os vowed to use the office of the Attorney General's office, if elected, to enforce the anti-trust provisions of the Texas and U.S. Constitutions, especially in regards to insurance company pricing. His other billboard message during the 2006 Attorney General's race was: "INSURANCE GOUGERS - I'M COMING AFTER YOU"
A firm believer in U.S. and Texas Constitutional law, Van Os outlined his promises in a legal affidavit and filed it with the county clerk of each county in Texas. The 2006 Attorney General's race was the second time in eight years that Abbott and Van Os were statewide competitors. They had faced off earlier in a race for Justice of the Texas Supreme Court. In 2006, Van Os consistently challenged incumbent Abbott to a debate, which Abbott declined.
He married Rachel Barrios-Van Os, a native of San Antonio. They have one son, B.J. and three daughters, Kay Cee, Maya and Leya.
A political activist, Van Os attended his first Democratic State Convention in 1972 and attended every one through 2006. He served as President, Northeast Austin Democrats, 1978-1980, Democratic Precinct Chairman #132, Travis County, 1981-1988, County Democratic Chairman, Travis County, 1996-1998, Chairman, North East Bexar County Democrats, 2000-2003, Life Member, NAACP Member and Legal Panelist, American Civil Liberties Union, and Member of the Labor Council for Latin American Advancement.
Van Os served as general counsel for the Texas AFL-CIO from 1983 through 1989 and was named Civil Libertarian of the Year, Central Texas ACLU, 1990 and was In-House District Counsel for the Communications Workers of America in Austin from 1981 through 1984. He met his wife, Rachel, while they were on a CWA picket line. One of his prominent clients was Bill Burkett in the Killian documents affair. He is the owner and Senior Attorney of David Van Os & Associates, San Antonio Texas, firm of four attorneys concentrating in Constitutional law, labor law, and civil rights, 1984-present.