Theses and Dissertations can be found through various databases on Databases A-Z . There are specific databases for European theses. For international theses databases, use the Theses Libguide.
Oxford theses can be found on ORA (recent titles in full text) or on SOLO. The Bodleian Libraries website has a page on finding Oxford print theses.
Theses that aren't available at Oxford may be requested through Interlibrary Loan.