It would also help to know which Outback model you are asking about.
Engine oil, about 1.1 for the 4 cylinder and about 1.5 for the 6; Automatic Transmission Fluid about 2.2; Manual transmission little under 1; Engine coolant a little less than 2 for the 4 cylinder, a little more than 2 for the 6; gasoline almost 17; other capacities are far less than a gallon. Your owner's manual has all of this information.
Now if you are asking how much milk you can bring home you are more likely limited by weight than volume. There is a label on the driver's door pillar with gross vehicle weight ratings, figure 8 pounds per gallon and don't forget to include your weight.
The gas tank on the 2003 Subaru Outback holds 16.9 gal..
10
The gas tank on the 2009 Subaru Outback holds 16.9 gal..
The gas tank on the 2014 Subaru Outback holds 18.5 gal..
The gas tank on the 2011 Subaru Outback holds 18.5 gal..
The gas tank on the 2001 Subaru Outback holds 16.9 gal..
The gas tank on the 2005 Subaru Outback holds 16.9 gal..
The gas tank on the 2013 Subaru Outback holds 18.5 gal..
The gas tank on the 2012 Subaru Outback holds 18.5 gal..
The gas tank on the 2006 Subaru Outback holds 16.9 gal..
The gas tank on the 2002 Subaru Outback holds 16.9 gal..
The gas tank on the 2004 Subaru Outback holds 16.9 gal..