Download the latest `apt-rxx.exe` from http://download.osgeo.org/osgeo4w/release/apt/. If you like rename to apt.exe.
Open a CMD shell and:
{{{
SET OSGEO4W_ROOT=C:\Osgeo4W
apt setup
apt update
apt install shell
}}}

== Example Session ==
D:\>'''set osgeo4w_root=d:\test-osgeo4w''' [[br]]
D:\>'''apt setup''' [[br]]
`Root dir not found, creating d:/test-osgeo4w` [[br]]
`creating d:/test-osgeo4w/etc/setup/` [[br]]
`creating d:/test-osgeo4w/etc/setup//installed.db` [[br]]
`getting d:/test-osgeo4w/etc/setup//setup.ini` [[br]]
`...100%` [[br]]

D:\>'''apt install shell''' [[br]]
`to install:` [[br]]
`    shell msvcrt` [[br]]
`Fetching http://download.osgeo.org/osgeo4w/./release/shell/shell-1.0.0-5.tar.bz2` [[br]]

`...100%  d:/test-osgeo4w/var/cache/setup/http%3a%2f%2fdownload.osgeo.org%2fosgeo4w/./release/shell/shell-1.0.0-5.tar.bz2` [[br]]

`a01e6dcdc9e2d2c49424585aa3e1b44f  shell-1.0.0-5.tar.bz2 - remote` [[br]]
`a01e6dcdc9e2d2c49424585aa3e1b44f  shell-1.0.0-5.tar.bz2 - local` [[br]]

`Fetching http://download.osgeo.org/osgeo4w/./release/msvcrt/msvcrt-1.0.1-2.tar.bz2` [[br]]
`...100%  d:/test-osgeo4w/var/cache/setup/http%3a%2f%2fdownload.osgeo.org%2fosgeo4w/./release/msvcrt/msvcrt-1.0.1-2.tar.bz2` [[br]]

`195c192b98b7333e2d3f9e04717c2eb5  msvcrt-1.0.1-2.tar.bz2 - remote` [[br]]
`195c192b98b7333e2d3f9e04717c2eb5  msvcrt-1.0.1-2.tar.bz2 - local` [[br]]
`installing shell 1.0.0-5` [[br]]

`>mkdir "C:\ProgramData\Microsoft\Windows\Start Menu\Programs\OSGeo4W"` [[br]]
`>xxmklink "C:\ProgramData\Microsoft\Windows\Start Menu\Programs\OSGeo4W\OSGeo4W.lnk" "d:\test-osgeo4w\OSGeo4W.bat" " " \ "OSGeo for Windows command shell" 1 "d:\test-osgeo4w\OSGeo4W.ico` [[br]]
`>xxmklink "C:\ProgramData\Desktop\OSGeo4W.lnk" "d:\test-osgeo4w\OSGeo4W.bat" " " \ "OSGeo for Windows command shell" 1 "d:\test-osgeo4w\OSGeo4W.ico` [[br]]
`Post_install complete, return code 0` [[br]]
`installing msvcrt 1.0.1-2` [[br]]

`>"d:\test-osgeo4w\bin\vcredist_x86.exe" /q` [[br]]
`>del "d:\test-osgeo4w\bin\vcredist_x86.exe"` [[br]]
`>textreplace -std -t bin/o4w_env.bat` [[br]]
`Post_install complete, return code 0` [[br]]

D:\>'''<b>apt list'''` [[br]]
`msvcrt              1.0.1-2` [[br]]
`shell               1.0.0-5` [[br]]

----

Shell is only a skeleton, there are no applications installed. So to get something actually useful:
{{{
>apt available

 Packages available to install (* = already installed)

agg-devel                       mapscript-dev-python
apache                          mapscript-java
apache-manual                   mapscript-python
...snipped >140 package names...
libxdr                          xerces-c
libxml2                         xerces-c-devel
mapfish_framework               zlib

>apt install gdal
...snip...

>gdalinfo --version
GDAL 1.8.0, released 2011/01/12
}}}