Official comment

Yes, you can! Twitter Cards will work with all versions of ONTRApages!
First Step
Go to the Getting Started with Twitter Cards for the overview. Choose the Card Typeyou want to use. For this example we are using the Summary card type at https://dev.twitter.com/cards/types/s...
Copy the code on that page where it's nicely formatted, or use this code block:
<meta name="twitter:card" content="summary" /> <meta name="twitter:site" content="@flickr" /> <meta name="twitter:title" content="Small Island Developing States Photo Submission" /> <meta name="twitter:description" content="View the album on Flickr." /> <meta name="twitter:image" content="https://farm6.staticflickr.com/5510/14338202952_93595258ff_z.jpg" />
Second Step
Paste that code into your ONTRApages page on the Settings tab in the "Custom header code" section, as shown here:
Edit the code as shown, with your own Summary, your Twitter address for the "twitter:site" value, a "headline" for the "twitter:title" value, and your description in the "twitter:description" value. To add an image, paste the image URL into the "twitter:image" value.
Third Step
Save and publish the ONTRApage.
Fourth Step
Now, go to the Twitter Card Validator site and enter the URL to your ONTRApages page. The Validator will show you a preview of the Twitter Card:
Include the ONTRApages URL in a Tweet, find your Tweet in your feed, and click on the date / time to expand it. The card will show below the Tweet!
Other Twitter Card Types work too; here's an image of the "Summary Card with Large Image" Card Type:
Please Note:
Twitter Cards show up only when you expand the Tweet by clicking on the date or time. They do not show up in the normal feed. This is a limitation of Twitter and not a limitation of ONTRApages.