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. 

Definition of Open Access filters

All OA
A free version of the publication is available (publication is either Gold, Hybrid, Bronze or Green OA)
Gold
Publication is published in a fully open access journal (this includes all publications with a Gold OA status in Unpaywall and those on our own fully OA list of journals)
Hybrid
Publication freely available under an open licence in a paid-access journal (Unpaywall OA status = Hybrid)
Bronze
Publication freely available on publisher's website, but without an open licence (Unpaywall OA status = Bronze)
Green
Publication freely available in an OA repository (Unpaywall OA status = Green, or publication type in Dimensions = Preprint)
ClosedNo freely available version has been identified (publication is neither Gold, Hybrid, Bronze nor Green OA)