Seems to me like the exact same thing designed for slightly different reasons. 1) To relieve inherent pressure in the circuit while the machine is on but not in use. 2) To limit overall pressure while brewing. In the Oscar the opv or expansion valve was designed with the first purpose in mind. While the Oscar "expansion valve" does technically address the 2nd issue I don't think it was designed with it in mind otherwise it would have been placed on the cold side of the boiler. This is just my guess.

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