oath、pledge与vow这些名词均含“誓言”之意。

oath : 指对上帝或神发出的庄严、正式的誓言以示自己话语的真实性。现多用于法庭上。

• to take/swear an oath of allegiance
  宣誓效忠
• Before giving evidence, witnesses in court have to take the oath (= promise to tell the truth).
  作证之前，证人必须当庭宣誓据实作证。

pledge : 普通用词，指保证去做或不做某事的郑重许诺。

• a pledge of support
  支援的许诺
• Will the government honour its election pledge not to raise taxes?
  政府会履行它的竞选诺言，不增加税收吗？
• Management has given a pledge that there will be no job losses this year.
  资方保证今年不会削减工作职位。

vow : 指庄严的许诺或誓约。

• to make/take a vow
  立 / 发誓
• to break/keep a vow
  违反 / 履行誓约
• to break your marriage vows
  背弃婚姻誓约
• Nuns take a vow of chastity.
  修女矢发贞洁愿。