In order to only display results which are open access, go to the bottom of the filters on the left-hand side, expand the 'Open Access' filter and select 'All OA' by clicking on 'Limit to'. When this filter is applied, only publications that are open access will appear in the results list. As with all filters, this can be done with a blank search, or along with a keyword search or abstract search, to display results in a particular area, and can also be combined with other filters.

For example, to find open access publications from 2015 including the word "CRISPR", simply search with the keyword "CRISPR" in the search box, then select "2015" in the 'Publication Year' filter, and select the 'Open access' filter also.

Definition of Open Access filters

All OA
Freely available copy is available
Gold
Free copy available on the publisher page, with or without an open licence
Green, Published
Free copy of published version available in an OA repository
Green, Accepted & Submitted
Free copy of accepted or submitted version (or where version is unknown) available in an OA repository
Closed
No freely available copy has been identified