Bus,
The part 145 organisation contracted to maintain the aircraft at specified locations approves all the maintenance carried out on it's behalf. The QAM is responsible for issueing One-Off Authorisations - which (I believe) should be reported to the NAA when used.
A One-Off is supposed to be used where there is NO contracted support, although there is often an unwritten 'arrangement' to call a local organisation for this situation if an irregular/ad-hoc service is scheduled.
A One-Off should be used only when no 'contract' exists and a local company that operates "similar" types (e.g. a Twin-Jet aircraft) may be approached to aid diagnostics or remedial actions.
That situation leads to a round of faxes/e-mails/calls to establish the complexity of the task, the detail of the remedial actions required and the capability of any proposed local engineers (i.e. the engineers ability to read manuals, etc).
ALL work done under this system MUST be re-done or at least re-certified by a company approved engineer immediately on return to contracted base.
A failure to satisfy these needs may result in sending a company engineer to complete the task.
Hope this helps...