Can two A positive blood groups make an O positive baby?

Hmm. I think your question's Wording is somewhat deceptive.

Are you asking if two A+ parents can have an O+ child?

Simple answer...YES.

Sounds wrong, but in reality its true. It all has to do with chromosomes, the DNA you inherit from your parents. The only thing really confusing is the terminology. Here's how it works: For blood group genetics, each chromosome (2 in total, one from mom and one from dad) can be either A, B, or O (O being the absence of the gene), and since everyone has two chromosomes for this gene, an individual can be AA, AO, BB, BO, AB, or OO. Now, the only problem is that simple blood tests can't distinguish between an AA and an AO (or BB and BO), in other Words, you can't see whats not there

So if Mom is AO (detected just the same as AA) and Dad is AO then ~50% of there kids will be AO, 25% will be AA, and 25% will be OO. but through simple blood tests 75% of their kids will be A and 25% will be O.

The + vs - factor really doesnt matter here and will only cause confusion.

If sombody is REALLY worried about paternity issues, simple and ~100% tests can be performed extremly cheap (and sometimes free).