Is it possible to get admission into B.Sc Math with Computing with +2 Sc (CBSE) 86.2% along with the availability of a hostel seat

As a rule, for converting your percentage to CGPA CBSE Class 12th, you must divide your overall percentage by 9.5; 

i.e. CGPA = Percentage/9.5

The University of Oxford gives critical importance to the predicted scores of the applicants who appeared for the CBSE exams. 

These scores are important because, based on these, the University of Oxford rolls out the conditional offer letter. Indian applicants generally need a minimum predicted

A candidate failing in one of the five subjects of external examination is placed in a compartment in that subject, provided they qualify in all the subjects of internal assessment.

Grace marks in CBSE Class 12 is also considered when questions are excessively difficult, outside the prescribed syllabus, or if the paper is unusually lengthy.

According to CBSE, the passing marks in Class 12 are 33%. If any student is not able to achieve 33% marks and misses out by one mark or so, then the board awards grace marks to students.