A relatively recent innovation of the London market, Partner Designate (PD), is a reaction to the disaffection of candidates who are given a promise of partnership which never materialises. The key element of a PD role is the clear and objective criteria for the candidate to achieve partnership within a designated time frame (6, 12 or 18 months). PD can also be an actual title although this is not essential.
Despite originating in the US, and thus a mainstay of US firms in London, Of counsel has now spread to UK firms. Without question OC is the most confusingly used title but, suffice to say, it can be used to denote a transitional role.
Associate director/managing associate/counsel (etc.)
Individual firms have adopted various titles to denote those on the path to partnership. Like PD these titles usually have a designated time frame and can be considered the internal name for partners designate.