Can an O positive mom and an O positive dad have a child with another blood-type?
A child can have O blood group only when two recessive O genes from both parents are inherited. A person is said to have blood type A or B even when a person possess one copy of “A” or “B” allele along with one copy of O allele, while only those who possess two copies of O are said to have a blood type of O.

Rhesus factor determines if the blood type is positive (+) or negative (-). Blood type is considered as A+ve or B+ve or O+ve if they have rhesus factor. People who don’t have rhesus factor are considered to have negative blood type such as O -ve. An O negative blood type is considered to be a universal donor since it contains nothing that would appear foreign to someone else’s blood. People with AB positive blood type is considered as universal recipients as they will not react to A, B, or O blood, and can receive a transfusion from a donor with or without the Rhesus antigen.

Now coming back to our question: Can an O positive mom and an O positive dad have a child with another blood type?

In ABO blood group system, If your mom is O positive and dad is also an O positive blood type then the child will have O blood type. Let’s try to understand this based on how alleles are passed on from parents to children.

http://www.scienceofbiogenetics.com/what-is-genetic-drift/