Barons had to swear an oath of allegiance to the king.

贵族须宣誓效忠国王。

《牛津词典》

Before giving evidence, a witness has to swear an oath.

作证之前，证人必须宣誓。

youdao

Don't I need to swear an oath?

我难道不需要发个誓什么的？

youdao

The cardinals swear an oath as they are casting their ballot -- further discouragement.

CNN: How secure is the papal election?

Presentation ceremonies are performed daily and recipients must swear an oath to the Queen and the Lord Mayor.

BBC: City limits - how free is a freeman?

Each man in turn stepped up and placed his hands on the Gospel to swear an oath in Latin.

BBC: Rome conclave: Cardinals begin voting for new Pope