A/B Testing with Optimizely (or any other split testing framework) is permitted by Google, but Google may crawl alternate versions of your pages
There are two ways of testing a page with Optimizely:
- Modifying elements on the page using the Optimizely Editor
- Specifying an alternative page to redirect a percentage of your traffic to.
Method 2 above is safe as well, although in order to prevent search engines from crawling both your original and your variant page (which is hosted on a different URL on your site, since you're using method 2) as duplicate content, you should use canonical URL tags as described here. (Thanks to Ruben Timmerman at Springest for pointing this out!)
Here is Google's position on the use of testing tools like Optimizely:
[A/B testing tools] are designed to keep your original content visible in the HTML source code of your page at all times. As a result, your original content is visible to crawlers, which means there should be no major impact on search engine ranking.