Just to give a sample and one of the ways to implement this:

I created an approval process, just added an action to update any field. In your case it will be your specific field.

[![DummyApprovalprocess][1]][1]


After that just wrote a trigger on Opportunity `beforeUpdate` like this. You can add your conditions.

    trigger OpportunityTrigger on Opportunity (before update) {

        if(trigger.isUpdate && trigger.isBefore) {        
            for(Opportunity s : Trigger.new){                        
                s.addError('You must add at least one osr Product to your Request');
            }
        }

    }

And when submitting for approval:

[![enter image description here][2]][2]


got this message:

[![enter image description here][3]][3]


Not you will have to fix your code for compilation errors and all, but this was just to show that this flow does work and allows us to put a validation in approval process.

  [1]: https://i.sstatic.net/WOKvm.png
  [2]: https://i.sstatic.net/VDOlZ.png
  [3]: https://i.sstatic.net/UBkbk.png