CHICAGO (AP) _ Temporary employees were on the job today at Oscar Mayer Food Corp.'s meat- packing plant here as hundreds of union employees remained locked out after rejecting a new contract.

About 45 new employees, most in sanitation and maintenance, began working at the plant Monday, said plant manager Allen Dieter.

He said the company would continue hiring about 35 or 40 people a day until the plant, which packages bacon, lunch meat and weiners, reaches full operating capacity. It was only at 5 percent to 8 percent capacity on Monday, he said.

Members of Local 100 of the United Food and Commercial Workers International Union, which represents 685 Oscar Mayer employees, voted 367-146 Sunday to reject a 10-month contract that would have frozen wages and benefits.

The Chicago workers, who receive $10.67 an hour, are among the highest-paid meat-packers in the country, said Steve Tarnowski, vice president of the local.

A few days before the vote, union spokesman Walter Piotrowski had said the company offer was an ''exceptional victory.''

But on Monday he said the contract was rejected because the union wanted a longer-term agreement, which could have ultimately meant that the Chicago contract would expire about the same time as contracts at Oscar Mayer plants in Davenport, Iowa, and Madison Wis.

If contracts at the three plants eventually expired about the same time, union representatives could negotiate together with the company, Piotrowski said.

Contracts at the Iowa and Wisconsin plants are to expire Aug. 31, and new contract negotiations are expected to begin about then, he said.

Growth of nonunion firms, employers' moves to decrease wages, bankruptcies and increased competition have caused the meat-packing industry to change in recent years.

The result has been a general trend toward cutting wages, Tarnowski said.

Oscar Mayer dropped its wage at the Chicago plant from $10.69 to $8.25 in 1984, but a federal appeals court ruled the cut violated the company's contract with the union and the old rate was reinstated. Two cents were deferred to other benefits.

The locked-out workers at the Chicago plant include 153 mechanical employees, among them electricans and pipefitters, and 532 production employees, Dieter said.

More than 600 workers were laid off work June 16, after two separate contract offers were made. Oscar Mayer had asked for a three-month wage freeze, and then requested a $10 hourly wage currently paid at the other two plants. The Chicago contract expired May 31.

No new talks are scheduled.