It gathers to a greatness, like the ooze of oil / crushed.
In this line, the alliteration is happening on the stressed syllables. ("Ooze of oil" is not quite a perfect alliteration, but it is close enough to have the desired effect, especially since visually, both words begin with the same letter.) The main effect in this line is to slow the reader down. The letter g is a heavily voiced sound, and the "oo" and "oi" sounds both require some effort to make, because they are long vowels and call for rounded lips. Therefore the reader has to slow down a bit on these words, which gives them a slightly greater emphasis and a ponderous dignity, just like something gathering to a greatness.
Why do men then now not reck His rod?
The alliteration in this line has a very different effect. It does not just slow the reader down, it actually makes the line very difficult to read, especially on the first pass. The rhyme of "men then" sets us up to expect few effects, or at least easy effects, after it ... but instead we immediately get "now" coming before "not," which trips us up. Then some difficult alliteration with r's, which is one of the hardest sounds in English to pronounce. This is called cacophony, and it is very appropriate to have a difficult, cacophonous line here because the poet is telling us that something is not right. The line also sounds really cool when it is mastered and read quickly and smoothly. Because "now not" interrupts the rhythm of the line, the alliteration in this line actually encourages us to move through it quickly, though trippingly.
... and all is seared with trade; smeared, bleared with toil,
In this line we have the unpleasant s sound in "seared" alliterating with sm sound in "smeared" (which is also difficult because we have to notice the addition of the m). We also have a lot of r's, near the beginning of the word in "trade," and near the end in the other key words. None of these words (except "toil") are particularly easy to say. The effect is like a tongue twister. It is, again, difficult to read aloud, and gives the reader the same unpleasant feeling that the poet gets when looking at this smeared, bleared world.