Is expiation (kaffarah) for breaking an oath incumbent upon someone who swore not to work in a specific job, then applied for it but was not accepted?

The reference point for oaths is the intention of the swearer and the reason for the oath. If your intention with the oath was specific to starting the job, not merely applying for it, then you have not broken your oath and no expiation is required of you. Breaking an oath only occurs by doing what you swore to abstain from, not by merely intending to do it.