A correspondent writes to ask if it should be disoriented or disorientated.
The answer partly depends on where you live. If you're American, you're in no doubt that it must be the shorter form; and according to Pam Peters (in her Cambridge Guide to English Usage) the same preference is found in Canada and Australia. Some US style guides go so far as to say that orientate is simply incorrect. That's going too far, as British English uses both, with a noticeable preference for the longer form. However, overall (globally speaking), the dominance of the longer form is evident: Google has orient 65 million vs 4 million, disorient 1.2 million vs 0.2 million.
The usage issue is relatively recent. For quite a while there was only the shorter form: the OED gives a first recorded usage for disorient in 1655, and for orient in 1728. The first recorded use of the longer forms is 1704 (for disorientate) and 1848 (for orientate). The new verb probably arose as a result of the associated nouns. Orientation (1839) and orientator (1844) preceded orientate, and the new verb usage would have been reinforced by the arrival of disorientation (1860). Certainly, by the end of the 19th century both verb forms were available.
Fowler has no separate entry on either word in his Dictionary of Modern English Usage (1926). We might think he would favour orient, because in his entry on 'Long Variants', he advises the use of shorter alternatives, as in prevent(at)ive, cultiv(at)able. On the other hand, in The King's English, orient is criticised as a 'Gallicism'. In his revision of Fowler's Dictionary (1965), Ernest Gowers (thinking of British English) suggests that orientate 'is likely to prevail in the common figurative use', i.e. with reference to goals rather than physical direction. This is an important distinction. We are more likely to say The course is orientated towards linguistics than The basilica is orientated towards the east.