Both Of My Parents Blood Group Is O+, But My Blood Group Is A. Is This Possible?

There is no way both of parents are your biological parents. Full stop. Because like already mentioned, people who have blood group O means they don't have antibodies for A nor B, they have no antibodies (lack of antibody expression on blood equals OO) Therefore how can you pass on genes to a child for a blood group that you don't have genes for yourself?

OO + OO

= OO

AO + BO

= AB AO BO OO

etc etc

I'm sorry if this causes you to become upset. Remember, parents are who bring you up and love you as their own, not whos genes you carry.

No it's not possible.  People with type O blood do not carry either the A or B markers.
You get one marker from each parent.  Blood type O is really OO.  Blood type A people either have markers of AA or AO.  Blood type B people either have markers of BB or BO.  People of blood type AB have markers of AB.

One of your true parents had to be blood type A or AB.

Sorry.

Father's blood group is b+. Mother's blood group is o+. Their son's blood group is o-. How it become negative? Is it possible?

No it's not possible, I have studied biology in graduation.

It is very strange for that to happen but I had a cousin like that so ide say yes hope it helps