He quickly took over the leadership position during the company crisis, proving to be a real opportunist.

opportunist 是由名词 opportunity（机会）加上后缀 -ist（表示"从事...的人"）构成的名词，字面意思是"利用机会的人"。在英语中常带有贬义，指"机会主义者"或"投机者"，指那些善于抓住机会为自己谋利的人。