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i was thinking when playing vs orks what should i fear
vehicals and squads of Nobs.
Army's used: Space Wolves, Tomb Kings, High Elves, Tyranids, Tau and Eldar.
Be aware that sending gaunts without support at squads of boys will not keep the boys busy for long. In equal numbers, ork boys tear spinegaunts apart.
And keep your precious MCs away from anything with a powerklaw.
Also, don't forget the bikers, who are very dangerous at taking out those big boomers that you have kept in the back ranks.
Also, lootas...People think that orks aren't good at shooting just because they have low BS scores, but man do they get a lot of shots for cheap!
Applaud, my friend, the Comedy is finished.
Hive Fleet Charybdis (1-0-0) and Grey Brothers Space Wolf Chapter (3-1-2) [W-L-T]
Being an ork player myself, along with recently getting into nids, I know from playing them from expirence that anything burny (burnas, kans w/ skorchas, burny buggies) is devastating against nids. As Ran49 said, PKs are dangerous against MC and pound for pound ork boys are pretty effective, so if you can try to get the charge against them. I'm probably committing heresy by helping out a non-ork player, but I always feel it's more fun to play against a smarter player who knows what to look for than just stomping all over someone who doesn't. Overall, remember to have fun win or lose and the best way to learn how to deal with a threat is through playtesting so if something's effective against you remember that for the next time.
If you can kill all lootas on the board and have a shooty-ish list you now have the shooting advantage. Use your VC/BS/DS/Devourers as much as possible until the orks get to you.
Here's your kill order at range:
1) Lootas: tons of high strength AP4 shots will kill your warriors & genestealers quickly. Use whatever you've got early on to make them run off the board.
2) Trukks: Orks without speed are far less threatening.
3) Battlewagons with deffrollas: Tyranids have no vehicles, so regardless of how people play the rule anything in the Tyranid codex is suffering d6 S10 hits. Everything from Gaunts to Carnifices dont want to be hit by this bad boy. Since they're open topped, your Venom cannons can have a field day on them. Besides, see the note for #2 above.
4) Anything with a skorcha, which will be skorcha buggies, looted wagons with skorchas, and if you've been following rules 2 & 3, trukks and/or battlewagons with burnas or kombi-skorchas inside.
If you happen to have a CC fex with S10, send him in a beeline at the Ork Warboss, as that will be the only thing that will insta-kill him. Otherwise you'll be eating a lot of S10 powerklaw attacks to the face and start losing your tyrants fast. Genestealers work wonders on most boyz squads simply due to # of attacks, and a 4+ save in return.
TL/DR version: Kill transports and smaller stuff first, especially if it has templates or multiple shots.
I should've said earlier, the trick I've found to dealing with orks as nids to exploit their crappy LD. Sure if they're in squads of greater than 10 they're fearless, but that's what shooting is for. Once they're below or near 10 guys, you assault them with a couple squads, or one squad that you know can get the job done. When you win combat, this isn't terribly hard considering everything in the nid codex aside from fexes are going to hit before orks, and they fail their LD check, you can very easily run them down and wipe out the rest, including that pesky nob.
I once had 4 CC warriors assault like 16 or boys with a nob, killed enough to get them below 10. The orks failed their LD, tried to run, and got mowed down by my warriors. Of course having 3 zoanthropes nearby doing psychic scream helped.