Originally Posted by Ap2020
You won't get from AO to Third Secretary on transfer. You will have to apply in the same way as anybody else must.
However, it is a great way to prepare for the role. They are ultimately equivalent jobs - and if you take on responsibility as an AO you will be able to fulfil the competencies. That's ultimately what those competitions demand.
This is very true, lot's of people do AO first as a stepping stone to Third Sec. As you say, very helpful for the interview if you have a year or two done as AO (especially if its in D/T or Finance).
Obviously a foreign language is also helpful and if you have worked abroad that will also be in your favour.