The best that my parents and me have tasted so far to date. I'll update that statement should I find somewhere else that has better tasting oyster omelette.
This stall is located at the hawker centre at Toa Payoh Lorong 6 Blk 22A, and it's unit number is #01-25 as you can see from a photo taken of it (at the bottom of this post)
It's omelette has a mixture of both its crispy parts and soft parts. As usual, it is salty, so a drink is usually needed. The stall sells this dish in several "sizes" — $3, $4, $5, and $8. I don't know about the other sizes, but for $3, the number of oysters is roughly 2-3 per dish, which I think is just right.
For those who do not like the oyster omelette, there is also the prawn omelette that one could order. It's exactly the same save for prawns substituting the oysters.
My family has so far only been there around evening, and so I'm not sure when the stall opens, but it stops taking orders rather early; around 7pm. So be sure to be there by 6+. When you place an order with the wife (the stall is run by a couple), she'll always tell you that you have to wait a long while. Ignore this. Typical waiting time I've experienced is around 15-20mins. While you are waiting, I'd recommend ordering dishes from some other stalls around, like the fried kway teow (cha kway teow), carrot cake (cai dao kway), and the satay, which are pretty nice too!