Objective morality is simply the proposition that something is moral independently of what anyone thinks about it (it isn’t subjective).
Theists typically say God’s nature is defined as “the basis” or “the foundation” for morality.
Disregarding that this claim is subjective opinion (I can simply disagree that God’s nature is the basis for morality, and then we’re just battling opinions), if we simply define God to be the basis for morality and it becomes objective by virtue of this (which must be the claim of theism), then we can define something else to be the foundation and it would be objective too.