Can I call my program “Open Source” even if I don’t use an approved license?

No, it’s unadvisable because you will interfere with the authenticity of existing approved Open Source Software licenses. When software developers refer to their code as Open Source without an approved license, it floods the tech community with unsupported software licenses; this makes it harder for others to decide which license is ideal for their projects. Fortunately, the Open Source Initiative has a process that reconciles and approves software licenses that meet the expectations and norms of the global software community. The process ensures a license goes through a public review and fits the criteria of an Open Source software before a committee approves it.