You can use the BYDAY property of the repeat interval for the scheduler. You can use this to set jobs to run on the nth weekday of the month. e.g.

FREQ=MONTHLY; BYDAY=1THU,3THU;

Or if you wanted the 1st Monday, 3rd Wednesday and 5th Friday it would be:

FREQ=MONTHLY; BYDAY=1MON,3WED,5FRI;

If you want it to run at a specific time on these days, use the BYHOUR, BYMINUTE & BYSECOND options:

FREQ=MONTHLY; BYDAY=1THU,3THU; BYHOUR=16; BYMINUTE=0; BYSECOND=0

To submit the job, run:

BEGIN
    DBMS_SCHEDULER.CREATE_JOB (
            job_name => 'TEST_JOB',
            job_type => 'PLSQL_BLOCK',
            job_action => 'begin your_code_here; end;',
            number_of_arguments => 0,
            start_date => NULL,
            repeat_interval => 'FREQ=MONTHLY; BYDAY=1THU,3THU;',
            end_date => NULL,
            enabled => FALSE,
            auto_drop => FALSE,
            comments => '');
      
    DBMS_SCHEDULER.enable(
             name => 'TEST_JOB');
END;
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