OSCP

Should learn:

• python
• buffer overflow
• port forwaring and pivoting
• practice post exploitation

Read Jollyfrog's tale
http://www.techexams.net/forums/security-certifications/110760-oscp-jollyfrogs-tale-5.html

OSCP-like Vulnhub VMs

Before starting the PWK course I solved some of the Vulnhub VMs so I don't need to start from rock bottom on the PWK lab. Below is a list of Vulnhub VMs I solved, most of them are similar to what you'll be facing in the lab. I've written walkthroughs for a few of them as well, but try harder first ;)

http://www.abatchy.com/2017/02/oscp-like-vulnhub-vms.html

Beginner friendly:
All these are *nix based

Kioptrix: Level 1 (#1) 
Kioptrix: Level 1.1 (#2) 
Kioptrix: Level 1.2 (#3) 
Kioptrix: Level 1.3 (#4) 
FristiLeaks: 1.3 
Stapler: 1
PwnLab: init
 Intermediate:
Kioptrix: 2014
Brainpan: 1
Mr-Robot: 1  
HackLAB: Vulnix
VulnOS: 2
SickOs: 1.2
/dev/random: scream 
pWnOS: 2.0
SkyTower: 1 
IMF

Windows
There aren't many Windows machines around due to licensing. Few options:
Metasploitable 3, will download a trial version of Windows Server.
https://github.com/magnetikonline/linuxmicrosoftievirtualmachines you can download Windows VMs legally then hack your way through them through an unpatched vulnerability or setting up a vulnerable software.

Set up your own lab. Default Windows XP SP0 will give you the chance to try out a few remote exploits, or doing some privilege escalation using weak services.
/dev/random: Sleepy (Uses VulnInjector, need to provide you own ISO and key.)
Bobby: 1 (Uses VulnInjector, need to provide you own ISO and key.)
https://github.com/g0tmi1k/VulnInjector

Scripts

Printing proof

Linux:
echo " ";echo "uname -a:";uname -a;echo " ";echo "hostname:";hostname;echo " ";echo "id";id;echo " ";echo "ifconfig:";/sbin/ifconfig -a;echo " ";echo "proof:";cat /root/proof.txt 2>/dev/null; cat /Desktop/proof.txt 2>/dev/null;echo " "

Windows:
echo. & echo. & echo whoami: & whoami 2> nul & echo %username% 2> nul & echo. & echo Hostname: & hostname & echo. & ipconfig /all & echo. & echo proof.txt: & type "C:\Users\Administrator\Desktop\proof.txt" 2> nul & type "C:\Documents and Settings\Administrator\Desktop\proof.txt" 2> nul & type %USERPROFILE%\Desktop\proof.txt 2> nul

Hints

Meterpreter is allowed on one machine. Normal reverse listeners are allowed in msfconsole. Msfvenom is therefore available.

If you cant find it on exploit-db its not the right path.