Because you gave "\\" in your addressing scheme; I know that your NAS server is for Windows platforms. It is using Microsoft's networked filesystem technology, known as SMB or CIFS, which are part of Microsoft Networking.
There are two packages which can be used. The one which is probably the best fit for you is sharity-light, which makes SMB/CIFS filesystems mountable on OpenBSD. The other package is Samba, which is a suite of tools which allow OpenBSD to participate in Microsoft Networking as a peer: a server for file/print, a client, a domain controller, and many other capabilities. One thing the OpenBSD package of Samba does NOT have is the ability to make remote SMB/CIFS filesystems mountable; which is one reason why I believe sharity-light may be a better solution for you. Another reason is that sharity-light is relatively simple to install, configure, and operate. The Samba suite requires a good deal of Microsoft Networking technical skill to configure and is a much more complicated endeavor.