No, your oyster card is not valid for your journey. Oyster validity in the Brighton direction only goes as far as Gatwick.
If you have a travelcard on your oyster card you can buy a rail ticket from the last station where your travelcard is valid or buy a "boundary zone" rail ticket and travel through on a single train.
Otherwise you would need to either buy a regular rail ticket for your whole journey or leave the train and touch out within the oyster area.
Govia Thameslink railway (operators of the "Thameslink", "Southern", "Great Northern" and "Gatwick Express" brands) have their own smartcard system called "the key". This can be used both to hold some pre-purchased rail tickets, and can also be registered for a service called "keygo", which allows pay as you go travel in some areas (including London to Brighton), but for the most part* only on services operated by GTR.
* It is also valid on some bus services operated by brighton and hove busses and metrobus and on a handful or GWR rail services.
I assume you mean the Brighton which is on the south coast of England.
If so, no.
You can start the travel on your Oyster Card, but when you reach the end of the validity area you will have to leave the train and buy a new ticket.
It will be easier to buy a ticket from your starting station all the way to Brighton, which will be easier and likely be cheaper.