Is the man considered an oppressor of the girl he abandoned, and is there anything incumbent upon him towards her, and what should he do regarding his fear that she might cause him problems at work if she learns of his marriage to another woman?

It is not permissible for a Muslim man to have an emotional relationship with a foreign woman outside the framework of marriage. He must hasten to repent. Do not pay attention to the woman you left pursuing you, for you are not wronging her by leaving her; she is the one who wronged herself. Her supplication against you is a supplication involving sin, so it will not be answered. If the other girl is religious and of good character, then marry her. There is no sin upon you if something happens to that woman because of your marriage to someone else. It is preferable to marry another woman outside the workplace if that is easier.