Searchable abstracts of theses from around the world are available via Proquest's Dissertations and Theses.
The EThoS service from the British Library also contains information about UK theses and there is increasing availability of full-text downloads, which are available once a library or individual has paid for digitization.
At this time Oxford theses are not available electronically via EThos but some can be found on ORA (the Oxford Research Archive).
You can search for Oxford DPhil theses on SOLO, including those in the Oxford Research Archive.
DPhil theses can be requested from offsite storage to the Radcliffe Science Library for consultation.
If you are unsure about the new system of placing requests, there are instructions on how to do this at https://libguides.bodleian.ox.ac.uk/solo/search#closed%20stack.