As long as you know your own blood type, your DH's blood type, and your parents and in-laws blood types (only if you and DH are A or B), you can figure it out pretty easily.
Since O is recessive, if you're both O, the only thing your kids could be is O. However, A and B can be either homozygous dominant (AA or BB) or heterozygous (AO and BO), which is why you need to know your parents types as well.
I teach this stuff and am a scientist by training, so I'll be happy to help if you need it!
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