In the UK you have to go on an approved CAA course to qualify as an OJTI and you also have to have a minimum of 12 months validation in the specialist area (twr/aps/area) in which you are going to instruct before you can be an OJTI at your unit.
At my unit, OJTIs are required to undergo further unit training with a Local Competency Examiner before they are allowed to instruct trainees. They are also formally examined at least once per year to maintain standards.
In the near future, it will be a requirement for each OJTI to undertake a refresher course with one of the UK's approved ATC Colleges, although, if there enough OJTIs to warrant it, this will be able to be done by an instructor visiting the unit