Chop em up & shoot em up!!
The truth is that when it come to killing Space Marines you need to consider their armour save of 3+. If you intend using weapons with AP4 you can expect that two thirds of the hits will be saved but if you can find yourself using AP3 weapons or better then you radically increase your chances of a kill as there will be no armour saves at all.
Orks have limited options to AP3 weapons: -
Boom Bomb S7, AP2, large blast - one use only
Burna S User, AP3 - expensive points wise
Buzzsaw, Sx2, AP2
Deff Kannon - S10, AP1, Massive Blast - Stompa
Kannon Shell, S8, AP3, Heavy 1
Kan Klaw, S+2, AP2
Killkannon, S7, AP3, Ordnance, Large Blast
Killsaw, Sx2, AP2
Klaw of Gork, S10, AP1
Kustom mega-blasta, S8, AP2, Assault 1
Kustom mega-kannon, S8, AP2, Heavy 1, Blast
Kustom mega slugga,S8, AP2
Mega choppa, SD, AP1 - Stompa
Power Klaw, Sx2, AP2 - every Nob should one
Rokkit Launcha, S8, AP3 - cheap / free upgrades, often Twinlinked
Shokk attack gun S2D6, AP2, Ordnance 1, Large Blast
Smasha gun, SD6+4, AP1 -Heavy 1
Supa-gatler, S7, AP3 - Heavy 2D6 - Stompa
Supa-rokkit, S8, AP3 - Heavy 1, Large blast - Stompa
Tankbusta bombs, S8, AP1 - Grab em where you can
Tankhammer, S8, AP2 - expensive at 15 points
Tellyport blasta - S8, AP2, Assault 1, Blast - expensive upgrade
Traktor kannon S8, AP3 - Heavy 1, Skyfire
Zzap gun, S2D6, AP2, Heavy 1
A BS of 5 means that one off shots need volume or re-rolls so twin linked is good. Blast templates are great and the bigger the better. Its also good to be using Assault weapons as Ork units will be typically on the move. The main focus is to get into combat where decent WS statistics can really count.
Speaking of big templates with low AP values one name keeps coming up ..... Da Stompa!
And just look at the size of some of those templates. How many Space Marines could you take out under the Massive Blast of the Deff Kannon? Me oh my - how I would love to find out, in a game!