Absence in registry of ships
There are no records of an SS Ourang Medan in Lloyds Register for 1946. You can find records of other ships with similar names - for example the SS OrangOutan!
Some of the stories about the SS Ourang Medan mention that an American ship named Silver Star heard the morse-code distress call and responded. Sometimes the Silver Star is described as a US Navy ship and sometimes as a US Merchant ship.
Lloyds register for a relevant period only contains mention of one ship with a name staring "Silver Star". A Canadian merchant ship, not a US ship. It's name was "Silver Star Park" not just "Silver Star". This ship had been sold and renamed by the time of the alleged sinking of SS Ourang Medan (1947, 1948 or 1952 - depending on which story you read).
Refs: 1, 2
Dutch Newspaper "De Locomotief" 1948
The story seems to originate in a 1948 report in De Locomotief.
De Locomotief also reported on this story on two later dates. So there are a series of three reports on this subject:
Translation courtesy of hdhondt:
Alongside this is the finish of the story about OM. The conclusion leads us to repeat that we have no further details about this "puzzle of the sea". It is not possible to give an answer to the many questions that arise from this story. The conclusion is that the whole story is a fantasy, an interesting romance. Against this is the statement from the author, Silvio Scherli of Trieste, who persists in assuring its authenticity
Absence of other records
Google engrams finds no mention of Ourang medan between 1940 and 2008.
It finds plenty of references to the Mary Celeste.
The Ourang Medan is certainly a far less well-known story but if it's story had substance, I'd expect something more to have been written about it.
Onus of proof
I'm not sure how much is myth or fact.
Proponents of this account have not firmly established any facts.
When a claim is made that something very unusual exists (e.g. a fairy who exchanges teeth for money), the onus is on the persons(s) making the claim to provide some reasonable evidence. It is unreasonable to expect others to provide evidence of non-existence. Therefore, until or unless some firm evidence emerges, it is reasonable to treat the claim as a myth.
Compare the case of the Mary Celeste of 1872. It seems unlikely that a similarly mysterious event in comparatively modern times (1947/8) would not be recorded more thoroughly in registries, newspapers or court records of the time.