For Year 12s there are 3 general Oxford University Study (taster) Days 15,16,17 March in a broad range of subjects. You must have a minimum of 5 A* at GCSE (7 for Medicine) to apply.
See http://www.pathways.ox.ac.uk/#y12study for more details and application. Deadline 31/1/16
Additionally St John's College, Oxford running Study Days across dates in late Feb and March for German, English and History prospective applicants. Deadlines late jan to early Feb. Details at:
www.sjc.ox.ac.uk/3958/Year-12-Study-Days.html
Also University College, Oxford have Study Days for Humanities, Mathematical and Physical Sciences across mid Feb to early March. First deadline here is this Friday (15th January). More at:
www.univ.ox.ac.uk/content/activities-and-events