It's not so much "Orks are all low-level psychic", so much as the Orks generate their own psychic field. And yes, if enough Orks believe something will be so, then it is so.
Note that it's a very variable thing; if two or three Orks believe that their Trukk will go faster because they've painted it red, then it will. If a lone Ork believes he can use his pistol to hop in the air, then he quite possibly could.
It's also worthwhile to note that Ork tech generally is only capable of functioning because the Orks think it can. As a more extreme example, if I gave an Ork a rock and told him it was a "speshul shoota", then it would indeed shoot something.
BTW, fluff-wise, what amount of punishment can a space marine take? Can he take a hit from a tank shell? Will a bolter round only punch through his armour if he's unlucky? Does plasma scythe through his armour like in the table-top games? Basically, can you give me a general idea of how resistant space marines are to the weapons in 40k. Thanks.
Generally, it depends. Astartes physiology is known to be incredibly resistant to the things that would kill a normal man ten times over.
Tank-shell wise, it's very variable. This is because of the rather fractured nature of the Imperium's technology level. However, there are multiple accounts of Marines taking multiple hits from autocannons in the fluff. Plasma gun wise, like several weapons (in particular las, plasma, and melta weapons), they have a variable yield setting. At full power, a plasma gun could easily vaporize a Marine. Note that that's not a "Marines are weak, hurr durr", so much as a "Plasma guns are uber, hurr durr".
Bolter-shell wise, it depends on the munition type, and somewhat the writer. For example, there are several examples of normal bolter rounds punching through power armor in Graham McNeill's Ultramarines novels, while in James Swallow's Nemesis, the Eversor assassin has a combination bolt/needle pistol, and multiple bolt rounds are unable to fully kill a renegade Astartes.
In general though, you need either a very powerful weapon, or a lot of relatively weak weapons. Like, massed lasgun fire, which will work primarily due to lucky hits on weak points, by virtue of volume of fire. Tau pulse rifle bursts are similar, though pulse rifles area bit better than lasguns.
