Papers available as open access are indicated in the search results by a link labelled, for example, Free Full Text from Publisher or Free Accepted Article from Repository. Clicking these links will take you to an open access copy of the paper. They are also marked by a blue padlock icon on the left-hand side.
Open Access Content in Web of Science: video from the Web of Science Group.
This video demonstrates some of the open access features in Web of Science. Note that it uses an older interface so some features may now look different.