It was an oversight on my part.

Meaning

Acknowledging a mistake due to carelessness.

Examples

• John: 'I forgot to send the email.'
• Alice: '<b>It was an oversight on my part.</b> I'll send it right away.'
• John: '<b>It was an oversight on my part</b>. I'll make sure it doesn't happen again.'
• Alice: 'I missed a deadline.'