Obama was 17 delegates short of reaching a majority of the 3,253 pledged delegates available in all state contests.
Oregon offered 52 delegates; Kentucky had 51.
Counting aligned superdelegates as well, Obama had a total of 1,915 and Clinton had 1,721, according to the latest Associated Press count. That placed Obama just more than 100 delegates short of the 2,026 needed to clinch the nomination.
Obama sought Monday to exploit McCain's ties to lobbyists, drawing a contrast between the Arizona senator's reputation as a reformer and his relationship to special interests in his campaign.
McCain recently adopted conflict-of-interest guidelines that led to the departures of several campaign aides due to their links to lobbyists.
"The fact is, John McCain's campaign is being run by Washington lobbyists and paid for by their money," Obama argued in Billings, Mont. "I'm not in this race to continue the special interest-driven politics of the last eight years. I'm in this race to end it."
McCain, in Savannah, Ga., dared Obama to follow his lead on the guidelines.
"We have enacted the most comprehensive and most transparent policy concerning lobbyist activities, and I challenge Senator Obama to adopt a similar policy."
Although Obama doesn't take money from federal lobbyists and political action committees, he does accept cash from state lobbyists and corporate executives interested in issues before Congress. He has had unpaid advisers with federal lobbying clients, and some of his campaign officials were lobbyists before.
Puerto Rico, with 55 delegates, holds its primary June 1. Montana has 16 delegates and South Dakota, 15.
Just before that final trio, Democratic Party officials are scheduled to meet to decide whether and how to count delegates from Florida and Michigan primaries that were held in defiance of the party's rules.
Clinton won both, but Obama kept his name off the ballot in Michigan and neither candidate campaigned in either state. Counting those delegates in some way could tighten the race but not, absent other surprises, tip the contest to Clinton.