You can search for journal articles in Google Scholar and use the "Find it@Oxford" link to access electronic versions. Google Scholar is the largest database of scholarly literature; it gives you quick, relevant results but sometimes too many, and it lacks search tools.
Advanced search tips
You might also want to look at: Why Is Searching the MLA Bibliography Better Than Searching Google?
You will also find lots of useful information about relevant resources in the following Oxford LibGuides:
|History - British||SOLO - Search Oxford Libraries Online
Finding and connecting to e-resources, e-books & e-journals
Special Collections (manuscripts & rare books)
Official Papers: Parliamentary, Departmental and International Intergovernmental papers
Oxford Research Archive (ORA)