Searchable abstracts of theses from around the world are availble via Proquest's Dissertations and Theses.
The Index to Theses provides information on theses from Great Britain and Northern Ireland universities as far back as 1716.
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. Simply search for "thesis oxford geology" plus any keywords if you are interested in a particular subject.
DPhil theses can be requested from the Bookstack or Remote Store using the "Request" tab to the Radcliffe Science Library for consultation.
If you are unsure about the new system of placing requests please watch this short video