You can use AND, OR operators, and brackets. If you want to use AND & OR statements together simply, wrap each group in brackets and follow with an AND.
For example, if you want to search for just for people who work for say Microsoft or Oracle then you would type the following:
Microsoft OR Oracle.
If you wanted to narrow it down to people who work for Microsoft or Oracle but in London or Manchester then you would use:
(Microsoft OR Oracle) AND (London OR Manchester).
Let's say you wanted to further narrow this down to people with PHP and MySQL skills then you would use like this:
(Microsoft OR Oracle) AND (London OR Manchester) AND MySQL AND PHP