a friend, someone you work with, or someone who does the same job as you in a different company, political party, etc.

The soldier was asked by his oppo what was up. 这个士兵的一个同伴问他是怎么回事。

His reclusive style contrasted sharply with the flamboyance of his oppo at BP. 他的低调风格与BP公司跟他相同职位人的浮夸形成了鲜明对比。