The Öhlins shock is one of the things that impressed me about the OSSAs. They are adjustable for rebound and compression damping.
You will need a spring compressor (or 6 to 10 hefty zip-ties) to remove the spring.
Öhlins USA will rebuild these shocks for a reasonable fee (about $180 in 2018).
OSSA said that one of the changes in 2013 was a different bottoming cone. The part number on the spring changed as well.
The spring on my 2011 bike is black, P/N 00281-09. It has a 46mm OD and is 125mm long. It is progressive with a rate of 64 – 78 kg/mm.
The spring on my 2014 bike is yellow, P/N 00281-64/65 L2711.
SplatShop in the UK sells a variety of genuine Öhlins and aftermarket springs.