Oxtail soup - oxtail tightened up during simmering?
A common braising time for oxtails would be on the order of 2-4 hours.

Oxtails are a tough cut of meat, with a lot of connective tissue made of a protein called collagen.  It needs to cook in a hot, moist environment in order to hydrate into gelatin.

This process happens over time and temperature.  Typically, braising is on the order of 200 degrees F (93 C), give or take, in order to get the expected tender product in the expected time.

If your oxtails have not become tender already in the 5 hours you have already given them, perhaps your slow cooker's low setting is not hot enough.  This is of special concern because you also want it to be at least 140 F (60 C) for safety.

I strongly urge you to use an instant read thermometer and measure the temperature at several places within the pot.  If it is not above the safety threshold, you should discard the contents, which would have been essentially incubating for the entire time.

If it is lower than the desired braising temperature, you can try your high setting, or transfer your braise to the oven (if the crock is oven safe, or in another covered oven safe pot such as a dutch oven).  Oven braising is usually done at about 350 F (180 C), which should bring the contents up to the desired braising temperature.

In any case, an expected cook time of 20 hours seems excessive by a factor of about 4 times.

Note also:  if your goal is to produce beef stock for the soup rather than tender, delectable meat, you want the temperature up around a slow simmer, which is about 200 F (93 C), although at the end, the flavor will have gone from the meat into the broth.  Even for stock making, I cannot imagine a cook time being required in excess of more than 8-12 hours.