These are the databases being searched by One Search. You can choose to include or exclude particular ones by clicking "Choose Databases" above the search box after doing a search. Below is an example of the box you will get.
Academic Search Complete, Communication & Mass Media Complete, eBook Academic Collection (EBSCOhost), ebrary, Film & Television Literature Index with Full Text, Gale Virtual Reference Library, GreenFILE, International Political Science Abstracts, JSTOR, Library Catalog, Literature Criticism Online, MasterFILE Premier, OmniFile Full Text Select (H.W. Wilson), Oxford Art Online, Project Muse, Regional Business News
In particular you need to be aware that some things are NOT being searched, including: individual ebooks (encyclopedias, dictionaries, Brill's New Pauly, Columbia Gazetteer, Early Western Korans, EEBO, ECCO, Literary Encyclopedia, Cambridge Collections Online, ACLS Humanities ebooks, Literati, MyiLibrary ebooks); databases (CAIRN, Generalis, Persee, Newspaper databases - Economist, Financial Times, Times, LexisNexis, ArtStor, BHA and RILA, IBA, Annee Philologique, Frantext, Feminae, CIAO, Literature Resource Center, MLA, Mideastwire, Middle East Newstand, IPSA, Pepweb, PsycARTICLES, PsycINFO).
So make sure to search some of these databases individually:
Literature (EEBO, ECCO, Literary Encyclopedia, Cambridge Collections Online, Literature Resource Center, MLA)
French (CAIRN, Persee, Generalis, Frantext)
Newspapers (Economist, Financial Times, Times, LexisNexis, Middle East Newstand)
Art (ArtStor, BHA and RILA, IBA)
Psychology (Pepweb, PsycARTICLES, PsycINFO)