If two parents are o+ can the child be a-?

No: Two parents with O+ will only have offspring that are O+. A child with A- blood will need to have at least one parent with A- or AB- blood.

Blood types: To have "O" blood type, you have to be "O/O"; therefore have no other "letter" to give to a child. So, two O parents can only have an O child. If one parent is O and the other is A/A or A/O or A/B then you can have a child with blood type A.