There are two "Limit to" functions in Dimensions - one appears next to the entity you want to filter by (the name of the organization, researcher etc. that you want to filter to), and the other appears at the bottom left of the page when you have selected filters to use.
To combine two or more entities of one filter group with a boolean OR, you select the respective options from the filter panel or typeahead and click on “Limit to” at the bottom of the page - more entities can be added afterwards. If you click on "Limit to" next to the filter option it adds the entity to my query with an AND.
Parentheses are added automatically, i.e. "Oxford OR Cambridge AND 2017" will be interpreted as "(Oxford OR Cambridge) AND 2017". Adding parentheses manually or editing operators is not possible - i.e. it is not possible to change the above query to "Oxford OR (Cambridge AND 2017)".