You probably need more light. Oregano grows like a weed once it takes off. Have you compared it growing outdoor and indoor. I personally have a harder time growing food/herbs indoor.
I didn't compare the growths since they were practically all the same amount. It'd be a minimal difference. There was plenty of light in all cases though, I guarantee that wasn't an issue. Unless it needs more than full sun.
Try growing in well drained soil rather than compost, most herbs grow in reasonable poor soils.
I tried that doing that at first. I put it outside the garden, beside where we usually grow our flowers. It's not terrible soil, but definitely not filled with compost. After noticing it wasn't growing, I tried planting some in our garden. Our garden isn't exactly the best either yet, it drains pretty quickly.
Parsley is a cool season crop which will either bolt or die off in hot weather. It will also tolerate less sunlight than some other herbs.
Oregano, on the other hand, loves warmth and sun.
Both should be grown in lean soil (fertilizer results in big, but tasteless, plants) with enough water to keep the soil just slightly damp.
There are several varieties of oregano, and of parsley. Which ones are you growing? Greek oregano seems to be the easiest to grow, and flat-leaf parsley has more taste.
Perhaps that's why the parsley wasn't growing for me then. I was trying to grow it in our fairly warm summer.
I actually had worse luck with the oregano so I'm not sure why it was unhappy.
When I tried to plant them in pots, I used potting soil I believe. While not the best, it's usually what I start the plants out in.
I'm pretty sure the parsley was flat leaf "Dark Green" parsley. The oregano I'm not so sure about though. If I remember correctly it just said "oregano" on the package. I seemed to have misplaced all of my seeds from last year though. I wonder if I left them in the greenhouse on accident...