You can search for Oxford MPhil and DPhil archaeology theses on SOLO. Simply search for "oxford thesis archaeology" plus any keywords if you are interested in a particular subject.
DPhil theses can be requested on SOLO from the Bodleian Bookstack to the Weston Library for consultation.
Dissertations, portfolios and theses written by Continuing Education Archaeology students are available for consultation in the Rewley House Continuing Education Library . Recent years are shelved under the stairs in the Lower Library, ask staff for older titles. A list of titles held is available in the Library.
Index to Theses searches for theses across UK universities as far back as 1716
EThOS is a British Library project to digitise and make available UK theses.
At this time Oxford theses are not available electronically via EThos but some can be found on ORA (the Oxford Research Archive).
US Dissertations and Theses
Dissertations and Theses contains 2.7 million records dating back to 1637, some of which are available in full text. The Humanities and Social Sciences Full text collection will probably be the most useful.
For more information about finding theses, see the Bodleian Libraries website.