“I also understand that there is a need for the OU-D hechsher.”
Of
course there is. Even people who are stringent regarding “chalav yisroel” eat certain things that are not. A neighbor of mine eats non CY Yogurt, although milk and cheeses is exclusively CY. If an item contained yogurt and was dairy, he would want to know. The yogurt also must be kosher. Why an OU-D appears on a container of milk, was once expplained to me by an employee of the OU. A company pays for the certification and all products in their line that meet the OU requirements get the symbol. Even the milk. It neither adds or detracts from the status of the milk.
“I heard a seminary girl tell me that her school told her not to eat from a certain hechsher because it is the equivalent to OU,”
Go into Boro Park or Williamsburg and you will find many people who do not eat products with an OU. Sometimes it is due to kashrus concerns (EG: The way certain iced tea companies cool down their cans after pasteurization, is problematic according to some opinions if part of the product mix is non kosher grape flavor iced tea – or any non kosher flavor – grape being the most common) but mainly it is because of politics. Dont sweat it. Follow your parents and your Rav, not an ignorant seminary friend.
“I completely trust that the OU is kosher, because I grew up eating it my whole life in the states, but why would Israelis think their hechsherim are much better than the ou”
I think the OU is better than many hechsherim, including Israeli and “heimish”, but that is because I am familiar with the business (and rest assured, it IS a business).
“especially if they have to eat something when recruiting in the states for their seminaries, its not like they bring a suitcase full of three weeks worth of food. That would be kinda illegal.”
This is not really a concern. One can live their entire life in the USA and not eat a single food product labeled with an OU. What they dont know however, is that in the background their own heimishe hechsher relies on the OU. This is true for many commercial products (flavorings and the like for example) where the heimishe hechsherim have very little involvement in the certification process and certify very little in the way of products.