Well one of my main armies is Orks so I will share some taktiks for use against Space Marines and Chaos Space Marines.
1. HUG COVER I cannot stress this enough 30 orks in the open are going to be gunned down really qucikly depending on deployment, guns etc. also try to get most of your army behind cover it will save many of your boyz lives.
2. Power Klaws I love power klaws it rips apart armour, instant deaths most infantry. So yeah sure they might be a bit pricey at 25 points but for your boyz nob it gives 5 attacks on the charge at strength 10!
3. Even though I don't use them I will mention Kustom Force Fields, well let us ork players face we have crap armour saves so how about using you get a 4+ cover save from it.
4. Trukks if you have them use them I usally run this:
Shoota boyz mob 66
11x boyz
nob 10
power klaw 25
big shoota 5
'ard boyz 44
trukk 35
that's 185 points, sure if it gets blown up your in trouble but if it gets there it kinda owns.
5. Deployment make sure if you win go to the most cover heavy side to maximize your chances of surviving.
My list against my friends chaos:
HQ: Warboss
power klaw
twin-linked shoota
'eavy armour
cybork body
Elites: Nobz
big choppa
power klaw
'eavy armour
bosspole
Troops: Boyz mob
30x ork boyz
3x big shoota
Boyz mob
30x ork boyz
3x big shootas
Shoota mob
11x ork boyz
1x nob
power klaw
bosspole
1x big shoota
trukk
'ard boyz
Fast: Warbiker mob
2x warbiker
1x nob
big choppa
bosspole
stikkbombs
Deff koptas
2x deffkoptas
buzzsawz
twin linked rockets
Thats close to 1000 points I just haven't calculated it though.