Is it possible to make VSO (Visual Studio Online) repo be public visible?

At present, no. VSTS is not currently intended to be a public code repository like GitHub.

Update

As pointed out in the comments, public projects are an upcoming feature. https://blogs.msdn.microsoft.com/devops/2018/04/27/vsts-public-projects-limited-preview/

You can now change project visibility to public at Azure DevOps. First you set organization settings->Security-Policies -> Allow public projects and then set project settings. It's well described at current Azure DevOps documentation here and here