How do I determine \$R_o\$ for an op amp?
The first op-amp I could think of (namely the AD8605 because I use them a lot) has this graph: -



It also says in the tables that Zout is typically 1ohm (unity gain closed loop)

If you are looking at dinosaurs like the 741 then maybe it isn't detailed.

So, maybe you just are either looking at poorly specified op-amps or you need to look at the graphs and figures after the tables. TI tend not to state it explicitly but ADI do.