What are duties for jobs like Professor, Lecturer, Research Scientist, Research Assistant, and Research Fellow in the UK (Oxford)? In Oxford's job website (here and here), I see positions including Professor, University Lecturer, Departmental Lecturer, Research Scientist, Research Assistant, and Research Fellow. What are the differences between the roles/duties and salary between these?

These job titles are discussed in answers to Translating British faculty titles into American equivalents but only in terms of career progression,  without much detail as to roles, duties, and salaries.
It think it does vary from institution to institution. From the perspective of my University (and from my own job hunting in the UK), I would make the following observations. Salary pay points correspond to my University's interpretation of the national HE pay scale http://www.ucu.org.uk/index.cfm?articleid=2210.


"research assistant" implies fixed-term contract work on a research project (research only), where the postholder does not hold a PhD (and is not close to completing one), pay point 23.
"research fellow" implies fixed-term contract work on a research project (research only), with the postholder holding a PhD. Pay point 29 as starting salary; pay point 37 for a "senior research fellow", who might also be expected to supervise students and write grant proposals.
"lecturer" implies a permanent contract with the postholder expected to conduct research, teach, and write grant proposals. Pay point 37 starting salary; pay point 44 for a "senior lecturer", as a career progression and probably managing multiple PhD/PostDocs and bringing in more grant money.
"professor" implies a senior academic, probably starting at pay point 51, managing multiple staff and working on large grants.


I'm sure there are exceptions to this, but it does seem about right for the University where I work and the others I have applied to.