You can make your searches more complex with the wildcard symbols * and ?. Wildcards allow you to search for several terms with the same root, and are useful if you don't know exactly how your term is spelled.
Boolean searching combines multiple terms to search for a word. You can perform a search like this through Advanced Search (click on the image to view a larger version).
As you can see, an Advanced Search offers you several options through the tabs at the top of the search menu. You can perform your search in Entries (which will return a list of all the entries containing the term or terms you search for), Senses (which will return a list of all the individual senses within entries that contain your search terms - if a single word has two different meanings depending on the context in which it's used, each of those meanings is considered a separate "sense"), and Quotations (which will return a list of all the quotations in OED Online that contain your search terms).
The drop-down menu in an Advanced Search offers additional options:
Using the drop-down menus offered in an Advanced Search, for instance, you could perform a Boolean search like this, which would show you all the Shakespeare quotations indexed in the OED Online that include the word "love.":
The Advanced Search page also makes it possible to perform searches on variant spellings and within particular entry ranges.
With variant spellings, you can search, for instance, for an archaic spelling of a word that appeared before spelling was standardized. In the case of the entry below, searching for any of the listed "forms" as a variant spelling would bring up the main entry "remember":
Choosing the option "Restrict to entry letter or range" for a search, on the other hand, will limit the search results to those contained within a particular range of the OED. So if you're unsure exactly how a word is spelled, but you know the first few letters of the word, you can use this option to find all the words that begin with those letters and narrow down your search this way.