For one who completely denies a claims of debt: Bava Metzia 5a, 6a
Where the claimant denies debt, after being told by the court to deliver payment: Bava Metzia 16b-17a
Where the claimant denies debt, after being told by the court that he owes X money: Bava Metzia 17a
Requiring the claimant to swear, if the defendant isn't credible: Bava Metzia 5a-b, 16b-17a
Where a worker was told to collect his salary from a merchant, and now both merchant and worker come to the Employer demanding payment/reimbursal: Bava Metzia 6a [I am uncertain that this is an "Oath of Contradiction"]