[en] The puzzle of three-dimensional magnetic interactions in the structurally two-dimensional layered-oxide Na_xCoO₂ is addressed using first-principles calculations and an analysis of the exchange mechanisms. The calculations agree with recent neutron results, favoring the antiferromagnetic stacking of ferromagnetic planes. The superexchange via direct O-O hopping and through intermediate Na sp² hybrids couples each Co to its nearest and six next-nearest interplanar neighbors via equivalent paths. The individual exchange constants are rather two-dimensional, like the lattice itself, but due to multiple c-axis exchange paths, the magnetism becomes effectively three-dimensional.