Now when you add a new ad it will get the status inactive (if you setup the setting to approve the ad first).
But on the user listing page the user can manage his posted ads and can set a ad as inactive.

Only problem is then that the ad also get's the status inactive, the same as the ads that are awaiting approval.

That could be confusing for the user because you can't see the difference between the ads that you have marked as inactive and the ads that you have posted but waiting for approval.

Maybe a new status like 'Pending' needs to be introduced in Osclass so that the ads that are newly added and waits for approval get the status 'Pending' and the ads that you set as inactive hold the status 'Inactive'.

You must use plugin for admin validation , ie item  validation plugin.

I use that plugin "Item validation plugin" only when I add a new listing I see the status "Inactive", while it's pending for validation. The same status as the listing I marked to be "Inactive".

Where in the "Item validation plugin" can I set a different status for the listings that awaites validation?

Well, status is only Active or Inactive