OPC Write calls can contain multiple nodes/values to be written, and that's what we end up doing when you call WriteMultipleValues (we first split the arguments by the target server or other factors that demand splitting, but that's not your case I think).

OPC specs do not, however, prescribe that the order of elements in the array is the same in which the server should perform the Write.

Therefore the only way to achieve an ordered Write is to sequence it yourself, by making smaller calls (or even node by node, if that is what the requirement is), and only issue a next call when the first has completed.

There is also a special provision in OPC UA by which the server may indicate that the Write is returning the control to the caller, but the actual writing operation will be completed asynchronously later. If the server does this (probably not), then there wouldn't be any general way to achieve write ordering at all, because there is no completion indication.

Best regards