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 mélanger certains produits.
And when you return to France or to Europe... you feel like mixing certain products.
Caption 25, Le Journal: Gourmet en Bretagne
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 24–25, Le Journal: Saint-Exupéry – Part 1
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...
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 Food
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 29–30, Le Journal: Le Rôle de sa Vie
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!