Are we talking about Lippia graveolens? I don't see any reason why it wouldn't come true from seed since it isnt a hybrid. However, it does seem to be hard to find. Perhaps its difficult to germinate. It is a tropical shrub so buying a plant or propagating from cuttings is probably an easier way of increasing it.

In case you haven’t found your Mexican oregano yet, suggest contacting Pearson’s Herb Farm in Vista, CA http://www.pearsonsgardens.com/ They’ll surely have the plant and probably the seed, too.