Democrats took the offensive yesterday for the first time in their weekly radio battle with President Reagan, bringing in House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) to elevate and sharpen the dispute over who should be blamed for the condition of the economy.

In a taped message released three hours before the president's live broadcast, O'Neill charged that the administration's program "is not working because the program is not fair -- and just as important, because the people themselves know it is not fair."

It was the first time a Democrat of O'Neill's stature had taken part in the Saturday broadcasts and the first time the party launched a political assault rather than responding to Reagan's remarks.

Reagan, sticking with previous strategy in his five-minute live radio broadcast from Camp David, Md., ignored O'Neill's charges, even though they had been available in wire service news reports since Friday evening. A White House spokesman said Reagan had not seen the reports.

Instead, the president assailed what he termed "six big myths about the economy being spread by people who ought to know better." He said it is a "political fairy tale" that increases in defense spending have caused the huge federal budget deficit. Defense outlays dropped from 9 percent of the U.S. gross national product in 1960 to 6 percent in 1981 while non-defense spending nearly doubled over the same period, he said.

Reagan said it is not true that his administration "slashed social spending and caused a lot of human hardship." Even after adjusting for inflation, his administration spends three times as much for non-defense items as President Kennedy did in 1963, he said. Reagan said it was "a real whopper" to say his tax cuts boosted inflation, charging that "the real culprits" were those in Congress who voted against administration budget proposals. An "especially cynical myth" is that there will be no jobs until the recession is over, he continued. "Bad as current unemployment is, in most individual cases it's a temporary problem," he said.

Reagan said it is not true that America's best days are over or that there is no end in sight for the recession. "We're not out of the woods yet, but we're getting there," he said.

In his broadcast, O'Neill said Reagan had gone back on election campaign promises to preserve Social Security intact and not to use recession to reduce inflation. The two disagreed on the number of people unemployed, the president saying it is 11 million and O'Neill saying 13 million. O'Neill suggested that officials do not understand "the horror that lies behind those figures."

"Today, our economy is not only stalled; it is starting to slide backwards," O'Neill said. He added:

"It is not fair to throw millions of people out of work as part of a deliberate plan to slow down inflation."

"It is not fair to give huge tax breaks to the wealthy while denying the basics of life to those who need them."

"It is not fair to spend trillions on the Pentagon and, in the same breath, threaten to cut people's Social Security checks."

He suggested that the jobless be put to work fixing "a quarter of a million bridges that are below standard and miles of roadway that should be repaired."

Voters should send "the clearest possible message" to the administration in the Nov. 2 general elections, O'Neill said.

"Stay the course?" he asked, quoting the Republicans' campaign slogan. "That is unfair. America needs a change," he concluded.