Proving Grounds, a CTF environment maintained by Offensive Security, has shown to be a great place to help prepare the OSCP certification. Whilst Proving Grounds has over 70 boxes, there is a handful that you can concentrate on and are most representative of boxes you are likely to encounter on the OSCP exam.
Following on from the wonderful work completed by TJ Null and his OSCP Like boxes on HackTheBox, TJ has released what he considers to be OSCP like boxes on Proving Grounds.
Proving Grounds has both a free (Play) and subscription-based tier (Practice). To access all the boxes below, you will need an active subscription for their paid tier, Practice.
Be sure to check out TJ Nulls great blog on how to prepare for the OSCP.
Windows OSCP Boxes Proving Grounds
Linux OSCP Boxes Proving Grounds
Harder Boxes to Try