I agree with White Chocolate fully.
Also, if a nation hypothetically has to act either way with an ODP, whether it means backing the ally or not, how is the treaty in any way [i]optional[/i]?
If alliances are worried about others backing out on them, I'm sure there's a way to write certain limited mandatory clauses into an ODP. At that point, it's more about communication between the alliances and having a clear idea of what each clause of the ODP means. That's why every treaty actually contains clauses instead of just saying ODP, MDP, etc.