A police informer, one who betrays a criminal enterprise
A sex offender, especially one who is guilty of sexual offences against children.
A nonce word.
A pedophile.
A stupid or worthless person.
A value constructed so as to be unique to a particular message in a stream, in order to prevent replay attacks.
The one or single occasion; the present reason or purpose (now only in for the nonce).
One-off; produced or created for a single occasion or use. Denoting something occurring once.
A detective.
Opportunity.
Opus.
Operation.
Operative.
Operator.
An operator's license, usually ops