Opposition party leaders reached agreement tonight on a plan to oust Chancellor Helmut Schmidt on Oct. 1 and install a new center-right coalition in West Germany headed by Helmut Kohl, the Christian Democratic Party leader.

New national elections are to be held March 6 next year.

The agreement -- between Kohl, Franz Josef Strauss, head of the Christian Democrats' Bavarian wing, and Free Democratic Party leader Hans-Dietrich Genscher -- would appear to end Schmidt's chances of realizing his call for immediate elections and maintaining until then the minority government he set up Friday when four junior Free Democratic ministers resigned.

A brief communique issued after a 4-hour meeting tonight in Kohl's parliamentary office said: "The party and parliamentary chairmen of the Christian Democratic/Christian Social Union and Free Democratic Party advised their members in parliament to elect Helmut Kohl chancellor of the Federal Republic of Germany on Friday, Oct. 1, 1982.

"After several hours of debate," the statement continued, "they are acting on the assumption that timely agreement about political cooperation in the new federal government and the new majority in the Bundestag the lower house of parliament can be achieved, which will guarantee successful mutual action for solution of the most important tasks of German politics."

Initially, Kohl and Genscher had hoped to move against Schmidt this week, before Sunday's elections in the state of Hesse. That race has taken on national significance as a test of public sentiment about last week's coalition breakup. If the Free Democrats do poorly in Hesse, as opinion polls have been predicting, it could weaken their bargaining position in the ongoing negotiations about formation of the new Bonn government.

While Kohl had appeared willing to facilitate the rapid move against Schmidt this week, Strauss appeared more interested in increasing the pressure on Genscher.

As the talks opened this evening, they were overshadowed by the insistence of the Bavarian party chief that new elections take place this year. This put him at odds with Genscher who, with Kohl's support, was urging that a national vote be delayed until early next year.

Strauss' desire for a conservative government of Christian Democrats and his own Christian Social Union without the help of the small, centrist Free Democrats is widely known.

With the latest public opinion surveys suggesting that the Free Democrats in a national vote would be unlikely to draw the minimum 5 percent necessary for representation in the Bundestag, Strauss appeared anxious for early elections to eliminate Genscher, his rival for foreign minister and vice chancellor in a new government.

Ultimately, Genscher and Kohl got their way on the election issue. While Strauss came to the meeting with the unanimous backing of his party's executive for elections this year, Kohl's national party executive committee voted earlier in the day to accept an election date in the first quarter of 1983.

It could not be determined what concessions may have been, or may yet be granted to Strauss in return for giving way on the election date.

"We talked in a very oppen-minded atmosphere with each other," Kohl told reporters during a break in the talks.

Asked why the "constructive vote of no-confidence" against Schmidt was scheduled for next week, the Conservative party leader replied: "It is in the nature of the matter that we need a bit of time in order to really be able to conduct the necessary reflective, serious talks about future politics or policies."