I heard the following sentence:
Eso lo decide la embajadora.
As "eso" is the direct object of the sentence, the direct object pronoun "lo" seems redundant to me in this sentence. Is it used for emphasis? Would the sentence without it also be correct?
That redundant direct object "lo" is obligatory, because "eso" is well-defined.
El doblado del complemento directo antepuesto al verbo es obligatorio:
A mis hijos los veo sólo los fines de semana.
Esta carta hay que mandarla hoy urgentemente.
A mi padre lo despidieron del trabajo.
Sus cartas las guardó toda su vida.
Que se había casado conmigo por dinero lo noté sólo unos años más tarde.
Un error lo tiene cualquiera.
Un día así hay que celebrarlo.
Algunos temas no los hemos tratado todavía.
Source: Hispanoteca
The only case where "Eso decide la embajadora" might be possible would be in the unlikely case that "eso" meant "that kind of thing" used pejoratively, but not if "eso" refers to some specific matter.
Let's consider what the sentence would look like without the object pronoun "lo":
Eso decide la embajadora.
When the listener hears the beginning of that sentence, "Eso decide," it gives the impression that the sentence is going to be something like "That decides it [for me]," or "That clinches it." That would give the wrong impression, though, because that's not what this sentence is supposed to be about. The listener would have to switch gears midstream if the sentence were to start out as "Eso decide...."
We can avoid getting started on the wrong foot if we stick the object pronoun in there. With "lo," we know that "eso" is not going to be the subject of the sentence. We know that there is a subject coming, that we haven't heard yet.
You are right that sometimes a redundant object pronoun is included for emphasis (example). But I don't think that's what's going on here.
