File: ow_system_plugins/base/controllers/users.php
Function:  public function standardSignIn()
Try adding code after: 
OW::getDocument()->getMasterPage()->setTemplate(OW::getThemeManager()->.....

Following code structure in our software allows adding custom content into the head section:

For Page Titles:

OW::getDocument()->setTitle(OW::getLanguage()->text('prefix', 'custom_key'));

For Page Description:

OW::getDocument()->setDescription(OW::getLanguage()->text('prefix', 'custom_key'));

Try using OW::getDocument()->setDescription.

1. Add OW::getDocument()->setDescription(OW::getLanguage()->text('prefix', 'custom_key')); after the line specified earlier.

- replace 'prefix' with 'base'
- replace 'custom_key' with your unique key name. Ex: 'opengraph_for_signin_page'.

2. Save your changes.

3. Go to www.yoursitename.com/admin/dev-tools/languages and add your custom text key.
From first drop down  ( Section) you will need to select 'Base'.
Into the second input box (Key)  you will need to add opengraph_for_signin_page .
Into the third text area ( Text) you will need to enter your actual open graph code .

4. Click Add.

And you are done. Make sure to check whether this will work out or not for sign in page ( www.sitename.com/sign-in URL).

thanx. that is a longer version than what i did. I have it correctly displaying now. Is your step more of a permanent solution? 

I just edited the users file and added the meta lines i needed and it worked.

JoshWho,  my solution is more for multi-lingual websites.

Your solution will be passing one and the same description text no matter which language is active.

My solution allows adding translation for each language separately.

As server owner you can always edit your description with your source code in case you change the language. It is just easier to do so via Admin panel >> Settings >> Languages.

Overall my solution is more for admin's convinces in future. Not all Oxwall users are fond of editing source code very often.

I understand now.

Like a good 58% of my site is code edits to make everything work the way I want it. I'm use to working with the direct scripts then a cms like structure. Seems like there is a lot of stuff still missing from the admin panel to do most of the correct seo and css and java loads. It is a really good platform just needs some cleaning and optimizing. I expected that with a open source software.

Thanx for your help Allia.  :)