Very strong buffers, but also with a wide range of offensive skills. Their buffs either buff the whole party (Warcryer) or alliance (Overlord). They do, however, miss out on certain buffs, and they tend on average to get buffs at much later levels than their other counterparts.
They start out pretty slow, and then really come into their own at higher levels.
Soul Cry and heavy armor mastery basically make the orc mystics almost as good at melee as a melee class, and they have drains and DOTs (damage over time skills). Frost Flame/Chill Flame is a DOT that the orcs have that is uncurable by any method; it will always run its full course.
How are they at offensive actions? Weapons they use and how do they hold their own soloing, at least lvls 1 to 20 till the class changes?
This question comes up very, very often. And often the advice given is 'slanted' because the helper pretty much assumes your going to be multi-boxing a buffer and/or running in big parties all the time and then running off to PVP once you hit a certain level, because that is all they believe the 'end game' is for in L2. They also assume you have top equipment at every grade change because you are going to be twinking them.
Here's my advice/experience assuming none of the above.
1) L1 to L7 as a Orc Mystic is the absolute WORST grind in the game even considering starting dwarves. This is largely because you are a Mystic but you have NO starting spells, it's melee only until you can buy skills. Here is the one area where at least being able to twink top NG is almost a must. Instead of a 'windstrike' type spell Orc Mystics start with 'Soul Cry'. This is a toggle that increases patk by a set amount but drains MP per tick. The MP use rate is roughly equivelant to the 'Vicious Stance' skill that DD classes get. (High MP use)
2)L7 to L40. Orc Mystics really don't change at all or gain very many good buffs due to the first class change. Thus they are pretty consistent from L7 until L40. Although the skill costs will be a pain when you first hit L7. (If you can twink a high SP scroll though...do it!) For these levels there are two main methods of tried and true play. First, get a strong melee weapon and heavy armor and just melee and Drain mobs all the way up to L40. OR, you could get some robes and a high Matk weapon and do a Sleep/DoT combo then wack the mob a couple times to death. NOTE: As of C5 the orc drain skill costs 230% the MP a mages main 'nuke' does with pretty much the same power, but without any MP recovery skills or abilites...not even clear mind. So use it, but watch your mp when spamming it.
3 - Warcryer) L40+ Good buffs will be slow coming but they will be largely of the DD crit/attack type buffs. You can still solo with the melee/drain combo or the sleep/DoT, but pickup or clan parties are very worthwhile also. As you get more buffs, MP usage becomes more of an immediate issue.
3 - Overlord) L40+ Better buffs come late. Painfully so. But they do cap out sooner than the Warcryers. (All except for the few you get in the 70's) You continue to get the basic might/shield/evasion/acumen/mdef buffs, but your only added buffs will be the shield buff, WW and Beserk. The Sleep/DoT combo is no longer possible or useful. Seals are all extreme short range and fairly hard to use effectively until A grade. AOE is still possible, but its more effective to debuff with winter/chaos, etc and have a pole user beat the mobs down than it is to rely on the AOE DoT skills. (Using a bow or pole was nerfed badly in C5 for mystics.) Thus you are left with the melee/drain combo or clan/ally parties. And given the restricted nature of your buffs, it helps to have a decently sized party to make the most of them as your going to need at least another healer in addition to 1-2 others to make the party work well. MP use is again a big issue. You pretty much have to gear yourself for how your MP is used. The main difference between gearing a OL versus a WC, is the WC has the VR skill for added healing and the OL has to consider the added MP costs of Heart or added Drains.