From what I can gather, then using POSIX O_DIRECT with open() on blocking device files works just the same way as combining both FILE_FLAG_WRITE_THROUGH and FILE_FLAG_NO_BUFFERING with CreateFile()...

High-performance I/O generally involves the use of direct memory access
(DMA) operations.  With DMA, the I/O device transfers data directly to and
from main memory without the intervention of the CPU.  In the simplest form
of DMA, the controller is handed a pointer to a region of memory, given the
length, and told to do its thing.  The processor can then forget about the
operation until the device signals that the work is done.