Non-Members of Organic Group can still view Node links
Please allow me to offer helpful insights even 6 months after your posted question.  Hopefully, this will help others who are struggling with the Drupal module OG as well.

Note that in your content type, you should already have bundled Group content visibility as another field.  Go to /admin/config/group/fields to accomplish this:



In this picture as my live example, note that my Content Group is the Group content type and that the Content Group Submission is the content type FOR hosting articles that should only be visible to any particular group.  What you need to do is bundle Group content visibility as a new field for this content type as shown below.



This means that you will have Group content visibility field as an additional field to your own content type.

The Groups audience field is meant to make content authors specify the intended audience of the content they are authoring.  The Group content visibility field is meant to make content authors specify the exposition criteria of that particular content.  This is best shown in the following two pictures.  Let us now look at the field set of my content type Content Group Submission.  In my example, I went to /admin/structure/types/manage/content_group_submission/fields.



If you edit this field, you will see its values to be constrained to the following options by default: 

- None -, 
Use group defaults, 
Public, 
accessible to all site users


and Private - 
accessible only to group members.  

The last one appears to be what you are interested in.  You should make that as your default value for this field.



Save all of that.  And your content authors will be offered with these options.  In my case, go to /node/add/content-group-submission.



And this is how your content authors would be able to constrain your article's exposition to only his/her intended groups.  By this method, non-group members would not be able to see nor access the nodes (even titles only) that are intended for private groups where they do not belong.