Elements of an enforceable oral agreement:
1. "meeting of the minds" i.e. "I would love to train here", "Well, we really want you to come here".
2. An offer by one party with acceptance of the offer by the other, i.e. "we will rank you to match if you rank us #1" "OK, then I'll be ranking you #1".
3. The negotiation must involve mutual exchange of something of value, i.e. you get a resident, I get a residency.
4. Neither party is negotiating in bad faith, i.e. "heh, heh, heh, no way in hell am I ranking this program #1. I just want to be sure of matching," or on the part of the program: "we just want to be sure we still match within our top 10, in case we don't get the people we're REALLY ranking to match."
5. Performance by one side of the agreed upon action, i.e. "I ranked you #1, what the hell happened?"
In the Venn diagram of the spoken word, oral agreements and word of mouth occupy non-intersecting spheres.
And anyway, the NRMP specifically states that any such agreement is non-binding, and that for its participants, the only binding "offer" and "acceptance" is between the actual rank lists submitted by the parties.