Is it possible to have a child whose blood type is A if both of the parents are type O?

When talking about blood types you must consider the dominant and recessive alleles present. The alleles for ABO Blood typing are A, B, and O. A and B are both dominant over O and Codominant with each other. Meaning that when either is present with the O allele the offspring will present the A or B blood type respectively. If both A and B alleles are present then the person in question would have Type AB blood since both alleles are fully expressed. Since Type O is recessive it is necessary to have two type O alleles in order to express that phenotype.
Therefore you are able to get the following phenotypes: Type A, Type B, Type AB, and Type O. In order to have a Type O genotype expressed you must have a homozygous recessive genotype or (OO often written ii). The means that no other allele can be present.
Since children receive these alleles from each parent, and we know that both parents are Blood Type O we know that the child must have received the O allele from both parents. This means that it is impossible for the child to have any other blood type than O.
In order for the child in question to have type A blood one of the parents would have had to express the Type A allele in either an AA, AB, or AO genotypic combination. This would mean that at least one of the parents would have to be Type A or Type AB in order for this to happen.

A child's blood group (also known as blood type) is dependent on the blood group of his/her parents. The possible blood groups are A, B, AB, and O. Since there are 4 possible blood types of the mother and 4 possible blood types of the father, there are effectively 16 possible combinations of the blood type of the resulting offspring.
Each parent donates one of their ABO alleles to the offspring. Thus, a parent with O blood type will donate O, and a parent with a blood type AB will donate either A or B alleles. Using this analogy, if both the parents have a blood type O, each will donate O allele to the offspring and hence the resulting offspring will have a blood type of O.
Thus, if both the parents have O blood group (or type), the child cannot have a blood group A.
Hope this helps.