Outback consuming twice the fuel amount it should

Hello. So my outback has been consuming a lot of
fuel. It has been getting worse and worse over the
past two years. If had the front o2 sensors
changed and mechanic dies not be live it could be
the back ones as there is no carbon on the exhurst.  
Iv taken it to a tuning specialist but they also caent
find the problem I have already sunk a lot of money
into to try and fix it but nothing has.
Any help would be great.