I would generally advise you to practice online tests as well as books. To improve, I would make sure to look at the reasoning and thought processes behind the answer given (whether right or wrong) so you know how the paper ‘thinks’. Generally, I’d say the books I used were quite lenient, so makesure to practice online to, and twke any marks with a pinch of salt.
Oxford doesn’t have a minimum LNAT score per se, I think it’s just looked at in conjunction with your PS. Maybe someone else could advise?