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.