An accent, or the lack of one, can sometimes determine the meaning of a French word.
For example, let's take ou, the common conjunction that means "or." After his extensive travel abroad, Chef Rachel Gesbert likes to use exotic ingredients when he returns to France "or" to Europe:
Et quand on revient en France, ou en Europe... on a envie de certains produits.
And when we return to France, or to Europe... we feel like mixing certain products.
Captions 27-28, Le Journal - Gourmet en BretagnePlay Caption
Ou bien also means "or," plain and simple. Anglophones, seeing the extra word bien, might be tempted to translate ou bien as "or even," or to add some other nuance. But in fact, ou bien is used pretty much interchangeably with ou, as we find in the report on the recent discovery of Saint-Exupéry's lost plane, near Marseilles.
Mais personne ne sait s'il s'agit d'un accident, d'un suicide, ou bien d'un tir ennemi.
but nobody knows whether it's a question of an accident, of a suicide, or of enemy fire.
Captions 27-28, Le Journal - Saint-ExupéryPlay Caption
However, when we draw a simple accent grave over the u in ou, we get the adverb où, which is used to indicate "where." Anne Liardet, mother of three, racing solo around the world on the "Vendée Globe," tells us:
J'suis bien, là où je suis...
I'm all right where I am...
Caption 25, Le Journal - Les navigateurs du Vendée GlobePlay Caption
In their worldwide hit "Senegal Fast Food," Amadou and Mariam, the singing-songwriting duo from Mali, ask:
Dakar, Bamako, Rio de Janeiro / Où est le problème, où est la frontière?
Dakar, Bamako, Rio de Janeiro / Where is the problem, where is the border?
Captions 25-26, Amadou et Mariam - Sénégal Fast FoodPlay Caption
Another meaning of où is "when," indicating time. Notice the way French movie star Agnès Jaoui uses it when talking about dreams and fame:
C'est bien... de rêver, mais y a un moment où il faut juste se récupérer soi-même.
It's good... to dream, but there comes a time when you have to go back to who you are.
Captions 32-33, Le Journal - Le rôle de sa viePlay Caption
So, there you have it: the short story of ou!
FYI: Keep in mind there are at least two other words that sound exactly the same as ou and où, but have their own unique spellings: une houe is "a hoe," like we use in the garden, and du houx is "holly," the stuff the halls are decked out with come Christmas!