please guide me what are the chances of being successful in the sixth form oversubscription appeal?
No one can tell you that, I'm afraid.
There are very few 6th form appeals nationally - in part because I suspect many schools are failing to tell parents of their right of appeal.
As others have already said, the important thing is what the school's published admission rules
say about entry requirements for the 6th form. It is those rules that determine how places must be allocated, and any letter you are sent must be in accordance with the published rules.
The school would appear to be in breach of the Appeals Code which clearly states:
Notifying appellants of the right to appeal and the appeal hearing
2.5 When a local authority or an admission authority informs a parent of a decision to refuse their child a place at a school for which they have applied, it must include the reason why admission was refused; information about the right to appeal; the deadline for lodging an appeal and the contact details for making an appeal. Parents must be informed that, if they wish to appeal, they have to set out their grounds for appeal in writing. Admission authorities must not limit the grounds on which an appeal can be made.
2.6 Where a child is refused admission to a sixth form, they and their parents have the same right of appeal and where they appeal separately admission authorities must arrange the appeals so that they are heard together.