Kael has not really revealed much about the process of souls becoming angels, but a fair amount is known about the great machine of hell.
When mortal dies its soul becomes a Sluagh. If a God has a claim to a soul then he will normally send an angel or demon to collect it. Otherwise, the Sluagh may wander Erebus for years, perhaps haunting a place, trying to accomplish some task lest unfinished in life, or simply lingering long enough for an archmage to be able to resurrect it without the need to petition a god for its return to the world of the living. To the best of my knowledge only Camulos has granted petitions by living mortals seeking to resurrect souls from his vault, which he did twice, once for Charadon and once for Baron Duin Halfmorn, in addition to giving the soul of the Archmage Ram to Ceridwen for her to resurrect. Arawn has also allowed the resurrection of Artendain, but that was at the request of Sucellus rather than a mortal archmage.
Most Sluagh do not wander long, because it is very hard for the dead to resist the draw of the Wells of Creation. Normally they pass peacefully through these wells into Arawn's vault, where they cease to be immaterial souls but gain a sort of spiritual body that will live on forever in a world of their own dreams. Necromancers sometimes choose to live near a Well, because they can easily intercept the processions of souls and bind some to their will. It is not healthy for souls to linger too long. When Tebryn Arbandi closed the Well of Creation on the Sacred Isle of Nemora, souls were still drawn there but were unable to pass through, and their presence caused the once lush paradise to wither into the hellish landscape of The Dead Lands.
The Compact allows gods to claim the souls of those that worshiped them and were close to their aspects, whether they realized their devotion or not. (Most of Mammon's worshipers are greedy men who never realized their devotion to anyone but themselves.) The evil gods have become much more keen on claiming souls than the good gods. They work together, so that they can claim those who showed a modicum of each of their vices rather than only those who were devoted to a particular kind of evil. They will still claim some souls for themselves which will skip on to their own vaults, but most claimed souls are too weak and not corrupt enough to be of use initially these are sent through the whole machine of hell in hopes of forcing such pathetic beings into demon lords.
Souls that enter Mulcarn's hell enter it as Manes, the least of all demons, but demons none the less. They may either progress form there or wither away and loose their distinct identities to become part of a Tar Demon.
I think the souls that enter Mammon's hell are technically still Manes, but they seem to retain more of their old identities. Still, they have many of their memories wiped, which may happen repeatedly as those who are not greedy enough and do not show enough initiative to gather enough coins to escape are recycled though with no memory of being there before. Note that most actual evil men skip Mulcarn's hell and start here instead.
In Camulos' hell souls begin as Spinagon, which as pathetically weak creatures that have little more than the instincts of frightened and aggressive animals. They have all their mortal memories wiped, but presumably may gain them back somehow because when the Patrian Archmage Ram came here he knew quite well who he was and could even use a little magic by the time Ceridwen found him and offered him the opportunity to retunr to Erebus as Tebryn Arbandi.
Those who proceed to Aeron's hell loose what strength they may have gained over the course of their trials in Camulos's battlefields and become imps. These are weak minor demons that are trained to use cunning, and then magic, before ascending the ranks further. They have a very structured education in his great temples, where they must learn to respect their superiors and are assigned many humiliating menial tasks they must complete before matriculating through many stages of advancement. They are frequently forced to return to Erebus and serve human masters, but their real task there is almost always to convince mortals to sell their souls. After their advancement though the ranks they typically seek jobs working for the evil god whose precept they most embody, but the greatest of them instead proceed to Agares' hell as demon lords who sovereignty rule their own territories and constantly war with their peers in the vain hope of winning the admiration of the King of Hope.
In your evil orc example, your sluagh likely passes quickly into hell, possibly even bypassing the manes stage and starting as a spinagon in Camulos's hell. Your soul might technically be claimed by Bhall, but she does not have a hell of her own yet and so shares that of Agares. No one is allowed in the lowest hell, so the best (or rather worst) you could really hope for is the hell of the god closest to her precept, which I'd say is Camulos.
(I'm assuming that by evil orc you don't just mean your average dumb foot soldier, but someone like Orthus. I could actually see an orc like Rantine being claimed by Lugus, despite his piety to Bhall, as he does not show blind passion and rage but discretion and a desire for peace and the betterment of his people.)
I presume that souls claimed by good gods technically become angels immediately, but that it takes a long time for them to develop into what would be recognized as angels by most. The great assembly line of Hell makes powerful demons faster than the diverse heavens make powerful angels. Also, note that at any time it is possible for an angel to fall, and that it is also possible (although more difficult) for demons to rise. Even demon lords can have a change of heart, and choose to ascend though the machine in reverse by demonstrating the virtues opposite the vices of each level. Most who do try to repent get caught by Esus' hell, but some have made it out and found peace in Arawn's vault, or taken up a position as an angel in some heaven. Sirona is the most accepting of former demons, and it would not surprise me if many of her angels in training where assigned the task of entering hell to offer forgiveness to any demon willing to accept it, and to guide them through the perils that await the penitent in the places of perdition.
