 So oaths are ways of bolstering one’s claim to veracity by calling on God to witness your claim. If your claim is about your future conduct, then the oath can be understood as a promise, with God as the promisee. You can invite god to witness alone, on the tacit understanding that he sees and knows all and will take account, or you can invite him to punish you for falsity directly, e.g “May God strike me down” or “God blind me” (“blimey”).