May my expert attend the other side’s fact witnesses’ depositions?

Answer: Unless someone gets an order invoking “the rule” at depositions, there probably is not a problem. See: Pryor v. Estate of Edwards, 1991 OK CIV APP 49, 815 P.2d 202: “Rule” does not apply to deposition, absent a court order, which rests w/in trial court’s discretion. (Cites federal cases pro and con.)

The federal cases cited in the state case will give you a pretty good clue to which way the federal courts will likely rule. I am unaware of a 10th Circuit or Oklahoma Federal District Court opinion on this and the federal cases in other circuits seem to go both ways.