The Old Man
As we discuss in "Character Roles," the old man functions primarily as a guide for Orestes. The main guidance seems to be: "Stop standing around and talking and get to action already!" This is what he tells Orestes in the first scene of the play, when they've just arrived in town, and it's what he reiterates again after Orestes unites with his sister.
Interestingly, this message can be seen as one of the main themes in the play. Electra, after all, has been reduced to inaction and waiting since Agamemnon was murdered. She hasn't taken action at all. Until the moment when Orestes actually stabs Clytemnestra, he does a lot of gabbing, and needs to be reminded of the task about hand. Of course, his arrival in Mycenae is in itself more action than we ever get from Electra.
That Orestes does commit the murders is not only dramatically important, but thematically significant as well. Talking about revenge doesn't mean anything if you don't actually get around to doing it, which Orestes does. The repeated advice from the old man – "Stop talking! Start acting!" is therefore an important message to take away from Electra as a whole.