When you open a conversation on whatsapp and the contact is seen as online have they opened your conversation or just any conversation?
You can read from the WhatsApp FAQ:
What does "last seen at..." and "online" mean exactly?
"online" means that contact has WhatsApp open and is connected to the internet. However, it does not necessarily mean they have read your chat.
That clearly stats that a contact with "online" status just needs to have the app opened within an active Internet connection.