- (cryptography) one-time pad
- (fandom slang) One true pair (or pairing), the romantic pairing preferred by an author or reader of fan fiction.
- (electronics) one-time programmable, for electronic devices that can only be programmed once.
- On the phone.
- One Time Password (A password that is valid for only one login session or transaction).
- Open Telecom Platform.
- Opposite Track Path.