How is it possible for a mother and father with O+ blood type produce an A+ blood type baby?

It is not possible for parents who both have O blood types to have an offspring with a blood type other than O.

Blood types are due to the presence of agglutinogens on the surface of a blood cell. There are two types - A and B. There are four possible blood types, then, depending on the presence of these agglutinogens - A if only A is present, B if only B is present, AB...

It is not possible for parents who both have O blood types to have an offspring with a blood type other than O.

Blood types are due to the presence of agglutinogens on the surface of a blood cell. There are two types - A and B. There are four possible blood types, then, depending on the presence of these agglutinogens - A if only A is present, B if only B is present, AB if both are present, and O if none of the two are present. 

An offspring's blood type is determined by the parents'. Each parent has two genes determining which agglutinogens are present. For instance, a parent with a blood type O can only have genes that don't have any of the agglutinogens (because if one expresses A, then the parent will have a blood type A, which is the phenotype, and a genotype AO, in this case, O can be seen as a recessive trait -- AB blood type is possible because A and B are "co-dominant"). If a parent has a blood type B, on the other hand, two possible genotypes are possible - BB or BO (again, because O is recessive).

Hence, if a father and a mother BOTH have blood type O, they're genotypes are both OO - that is, none of them can carry an A or a B gene because the O blood type is a recessive trait. Hence, the offspring cannot have a blood type other than O. 

(A table in the first reference shows the possible offsprings given all possible combinations of genotypes of parents. Note that under OO for both, the only possible outcome is an OO -- which results into the phenotype blood type O).