That's exactly what happened to mine recently. I had mine growing in a communal self watering container growing between catnip and spearmint (silly me). It was doing fine initially, but the two mints grew gigantic and began blocking the sunlight the oregano was getting, which then started turning black. I got a new plant and put it in it's own container in a hanging basket and now it's really thriving.

I don't know if yours is the same problem. Has anything changed that could affect the amount of sunlight its getting?

Too much water and the crown rotted.

Thanks for your replies - at first I poured some peroxide on it and a couple of other herbs - I thought I would clean them up a bit. Nothing had changed as far as sunlight went, but I think the roots were getting really bound up on top of any other problem (which may have been too much water) that was going on. So, instead of losing them (I had 5 different kinds growing on a cart outside) I put them in the ground and they seem to feel much better now. It's a new experience for me to grow something in FLA. I was going by North standards on growing things and that is totally wrong for this area. Too harsh of sun, crazy bugs, nutrientless soil, etc. I'm surprised anything can survive down here!!!! I almost killed a cactus too.