With lobbyists filling the corridors outside, a House-Senate conference committee yesterday took up the $750 billion tax-cut bill to resolve differences over tax breaks for oil producers and commodity dealers, estate tax reductions and a dozen other sweeteners.

In preliminary action, the committee approved a new charitable deduction for taxpayers using the short form. It would be limited to $25 in 1982 and 1983, $75 in 1984, to 50 percent of total contributions without ceiling in 1985, unlimited in 1986, and require re-enactment thereafter.

In a significant development for working couples with children, the conferees agreed to raise the maximum child care credit from $400 to $720 for one child and $1,440 for two or more children. In stages, the credit would drop to $480 for one child and $960 for more when income exceeds $30,000. Cut out, however, was a provision that would have given working poor parents a direct federal payment if they had no tax liability.

The Republican House bill had called for shortening the time a taxpayer had to hold an asset to qualify for capital gains rates from one year to six months. The conferees rejected this without discussion.

Two agrements favoring upper-income groups would lower the maximum rate on estates in excess of $2 million from 70 percent to 50 percent, and would establish a $75,000 exclusion on income earned by citizens working overseas; the exclusion would grow to $95,000 in 1986.

In addition, the conferees agreed to let persons 55 or older avoid taxes on profits of up to $125,000 on the sale of a primary home, instead of the current $100,000 maximum. All home sellers would have two years, instead of the current 18 months, to reinvest their profit in a new home to avoid taxation.

Another compromise would permit salaried employes now covered by company pension plays to contribute $2,000 a year to independent Individual Retirement Accounts. The amount, paralleling what a self-employed individual can set aside in an IRA, would not be subject to taxation until it is taken out of the account.

The basic elements of the tax bill -- across-the-board rate cuts, major reductions in business taxes and automatic tax cuts to adjust for inflation starting in 1985 -- have already been resolved in President Reagan's victories this week on the House and Senate floors.

The costiliest provision left involves tax breaks for the oil industry. In the House, a bidding war between the Democratic leadership and the Reagan administration for the votes of oil-state congressmen pushed the cost of these provisions up to $13 billion over six years, according to Treasury Department estimates, and to $16.9 billion, according to the Joint Committee on Taxation.

The Senate, normally more sympathetic to the oil industry, added sweeteners totalling $6.6 billion; members were prevented from voting more when liberal Democrats threatened to filibuster.

That threat was renewed yesterday by Sen. Howard M. Metzenbaum (D-Ohio) and five colleagues. They took the floor to warn Sen. Robert J. Dole (R-Kan.), chairman of the Finance Committee, who favors bigger breaks for the oil industry, that agreeing to the generous House oil provisions is sure to provoke a new filibuster.

"There is one thing about the oil industry, and that is they want more, they want more," Metzenbaum said. He was backed up by Sen. Edward M. Kennedy (D-Mass.), who described the oil companies as "an industry of gred for profits which are unjustified and unwarranted." Both senatoars are up for reelection in 1982.

The administration backs the House provisions and Dole waved a red flag in front of the liberals on the floor with the claim that "there is not one additional dollar in the House bill going to the major oil companies."

Instead, Dole noted that the add-ons in the House will largely benefit independent producers, whom he described as "mom and pop" operators. In fact, many of the independents make enough money that the tax breaks from only one of the House provisions will amount to just under $1 million a year.

As the conference entered the early morning, remaining differences centered on the tax breaks for oil, on tax breaks for commodity dealers, and on changes in employe stock option plans.