Everybody gets one from each parent. And you can only pass on what you have. O means zero, really, because nothing is there, but we call it Oh.
Two people with O can only give O on to their kids, so both parents with O have kids with O
But O can be hidden if you have type B or type A, so two parents with B and A can have a child with AB, O, A or B.
Two with B can have a child with B or O.
Two with A can have a child with A or O.
One with O and one with A, child has A or O.
One with B and one with O, child has B or O.
One with AB and other with anything else at all can only have a child with A or B, not O.
You inherit one from each parent. If your mom gave you B and your dad O, your blood type shows up as B, but there's a chance that you pass on the O to your child and not the B that shows up when you do a blood test. Does that make sense?
If you inherited B from BOTH of your parents, you can only give B on to your child, but you can't really KNOW for a fact that you don't have a hidden O to pass on (and neither can anyone else) unless BOTH if YOUR parents had AB and you ended up with B (because then no O could be hidden, as your parents can't have had a hidden O).
Now does that make sense or have I just rambled on and confused you?