Re: O+ Father AB+ Mother, what could the baby's blood type could be?

Normally, based on the genotypes that you have listed the baby will either 
be "B+" (genotype BO) or A+ (genotype AO).  So usually the answer to your 
question is no. However, there are anomalies to the ABO blood typing.  One 
example is the Bombay phenotype.  I do not know of any that are specific 
to AB and O blood type matches.  However, there might be one.

The website listed below gives you a table of parents and offspring 
blood types  http://www.divorcenet.com/paternity/abo-blood.html
This website discusses the rare exceptions to the rule 
http://www.bh.rmit.edu.au/mls/subjects/abo/resources/frequency4.htm

Hope this helps

Mike