Marjoram vs. oregano
Marjoram is oregano's calmer, sweeter fraternal twin. Oregano = zesty + peppery + lemony. Marjoram = delicate + floral + round. The two are often used interchangeably but if you get up in their mix you'll see some big differences.
Want to test the difference? Get a fresh sprig of marjoram and a fresh sprig of oregano. Tear an oregano leaf in half. Hold it up to your nose. Smell that piney resin? That jolt? It's sharp, isn't it? Almost one note.
Okay, wait a few minutes, then do the same thing with the marjoram. Smell the complexity? The spice is still there but it's perfumed, heady. Almost soapy. (If you use too much of it, that soapiness can take over a soup or sauce.)
So, can you use one in place of the other? Sure. If you use oregano when a recipe calls for marjoram (or vice versa), the flavors of the dish won't be wildly different. Still, we like to honor and explore the subtle character differences between the two.