With a C12 sign there's no oneway :)

On the wiki it reads:
C12, closed for all motorverhicles
motor_vehicle=no & moped=yes & mofa=yes & traffic_sign=NL:C12

Often there is no sign or other signs on the other side of the road.
In these cases use *:forward=* or *:backward=* on a small piece of the road.
Routing, you are allowed to turn around on the road or visit a house and then drive back.
(in extreme, to just behind the sign, hence the "small piece")
Give all modes of transport and the traffic_sign forward/backward. And take note to any under signs.

https://wiki.openstreetmap.org/wiki/NL:Overzicht_Nederlandse_Verkeersborden

With a oneway road there's a C3/C4 sign with the direction on flow of traffic and an under sign telling what type of traffic you can expect driving toward you (if any).
There should be a C2 sign on the other end of the road prohibiting any vehicles with an under sign telling what types are exempt.
This is nearly never the case with a C12 sign.


So in this case there's only a short segment of road where the C12 sign is that prohibits traffic from entering this road from the north.
In this specific case the topmost house is allowed to drive south from their driveway because they didn't pass said traffic sign.
This short segment only prohibits motorvehicles from entering this road from the north with the tags:
motor_vehicle:forward/backward=no & moped:<same>=yes & mofa:<same>=yes & traffic_sign:<same>=yes

It wasn't really tagged correctly by the 3-years younger me either by the looks of it.
It isn't that much work so I can change it according to the wiki if you want.