Can I delete data on my computer after transferring it to OneDrive?

I saw an answer to "Free Up Space" on another Microsoft Answers page that made no sense. The first contributor or moderator said use Free Up Space, and one by one it'll take them off your computer forever. However, the next contributor said it'll remove them from your computer, but if you open that file again, that's in the cloud only, now it'll be moved right back onto your computer. The two contributors are contradicting each other.