I would like to add an answer to this question also, even though the first answer was good, because I might be able to be helpful as well.
First, as an aside, this happened to me with some catnip seeds once many, many years ago, where I planted some catnip seeds and didn't know what to expect. My cats were completely uninterested in the plants that resulted. I began to doubt they were really catnip plants. Later I found out that they weren't catnip plants at all. I think they turned out to be lemongrass - which they say some cats like, I think, but my cats expressed no interest. I don't really remember exactly. But the point is, I had planted the seeds myself. It was very confusing.
The way I happened to find out they weren't catnip was this: my mother-in-law recommended drying the catnip for the winter. I did so and realized immediately that it did not smell right.
The point is that dried herbs, as mentioned by the previous person, have much more recognizable smells than fresh ones do. Since I agree with previous answer that you could have something similar to oregano but not exactly the same, like sweet marjoram, here, maybe you should dry it and smell it. It also does look a lot like oregano, though. It's hard to tell without drying it.
Drying it is easy, though. Have you ever done that? Dried some? To do so, you would only have to cut off some of that beautiful, plentiful growth and trim the little leaves off the stems. Wash the trimmed leaves and leave them folded into paper towels for 4 or 5 or so days to dry out completely.
This will enable you to identify them better by smell because when they're dried out, they should smell like you expect oregano to smell.
By the way, in my personal experience, if you grind up the dried leaves using something like a clean coffee grinder or clean salsa maker, they will smell even more recognizable - assuming they are indeed oregano.