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.

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:

got this message:

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.

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.

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:

got this message:

Now 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.