Hey, xotishaa! I know the wait can be rough! The update's still in development at the moment -- once that's complete, T-Mobile will get a test and start working to make sure the update's hiccup-free. We don't have a timeline to share, unfortunately, but I'm glad to read that you know the right place to stay up to date! Hopefully it won't be too much longer before we're able to start testing!
According to Sam Mobile, Samsung has already begun to roll out the Oreo Update under the firmware version G930FXXU2ERD5 and G935FXXU2ERD5 and should begin to pop-up in phones from the 25th of this month and onwards after Samsung having to halt the rollout due to major bugs that affect powerups. It has already rolled out in Korea and the UK. I even spoke with a T-Mobile Associate earlier today; he said that the update is released in batches and also said the same thing that tmo_marissa answered for you: there is no exact schedule for when the update will come. BUT IT'S COMING SO KEEP THE HYPE ALIVE!