Both parent has a blood type, can they have child with o blood type?

Yes indeed: Blood cells show A or B antigens - if neither are present the blood type is O. A parent will AO or AA genes will be Type A. Since the child gets one gene from each parent - if both parents are AO and both pass the O to the child -- then the child will be O. The options are AA, AO, OA and finally 1 out of 4 gets the OO - and is type O. pretty cool stuff huh?

Blood types: Yes they can. If each parent is actual AO, then the child has a 25% chance of being OO, which translates to a O blood type