Looking to change @ContextNode.OS_Description to @ContextNode."Node Group"

I would recommend you utilise core Orion Custom Properties, rather than the NCM Node Group field, which I would not be surprised was removed eventually.

The following is an excerpt from another post on the use of Custom Properties within Config Templates, which should point you in the right direction.

CUSTOM PROPERTIES WITHIN TEMPLATES

Custom properties are tied with nodes, and therefore can be referenced via the node parameter:

script BaseChangeTemplate (NCM.Nodes @ContextNode)
{
  CLI
  {
    show @ContextNode.MyCustomProperty
  }
}

Assuming we have created and filled in the AssetTag custom property for our devices, we can make use of e.g. it in the following way to change access lists:

script ChangeAccessList (NCM.Nodes @ContextNode)
{
  string @myip = '10.10.'
  @myip = @myip + @ContextNode.AssetTag         here we make the asset tag part of the IP address
  @myip = @myip + '.32'                         finish building IP
  CLI                                           commands within a CLI block are sent to the device
  {
    configure terminal
    no access-list 112
    access-list 112 remark This is a test
    access-list 112 permit tcp @myip 0.0.0.31 host 123.234.123.234 eq 445       include the IP with asset tag info in the access list
  }
}