There are lots of weights to consider when towing but the main one to affect you with the Omega and a braked trailer is the UK Law which requires that the actual laden weight of the trailer (trailer plus contents, spare wheel etc etc) must weigh less than the kerb weight of the car.
The kerb weights vary with model and trim but are generally in the 1700kg area.
If you meet this requirement then you have to look at Maximum Train Weight, axle weights (only a problem if car is fully loaded) and vertical load on towball / nose weight of trailer. (Check for a plate on the towbar, the Omega can handle 75kg iirc but some cheaper towbars are only certified to 50kg)
The Renaults can be absolute swines to work out the towing capacities. They are usually fairly light for their size which can be a problem on kerb weight vs trailer laden weight. If you get past this stage then read the handbook very carefully, they quote a tow weight based on a driver only so you have to deduct the weight of any passengers or additional luggage etc and its amazing how the limit falls. You still see Renaults towing large caravans etc but if there was an accident then the insurance company could refuse to pay on the grounds that the vehicle was being operated incorrectly.
I was looking at a 2l scenic once, kerb weight 1300kg, and worked out that with family and holiday luggage onboard then the tow weight was around 800kg, this seemed very low so checked with Renault UK who didnt know but got it confirmed by the Regie eventually.