Here are two points on Oppen’s poetry that have made a significant impact on how I think about my own work.
On the multiplicity of voices that speak in a single poem:
“Quotations are everywhere in Oppen’s poetry, especially in his later work, where he began to distinguish them by using italics rather than quotation marks, thus marking them not as speech but as another layer of text. Increasingly, the incorporation of ‘foreign’ materials does not point outside the poem, but functions rather to disrupt any sense of unified poetic ‘voice’ even though sources are often obscured.”
On poetry as a form.of knowing:
“The poet may recall a past experience, then, but that experience is fundamentally recast, perhaps so as to be almost unrecognisable, when caught up in the force-field of present perception. We are dealing not with a situation in which a given subject appropriates something other as an object of knowledge, but rather one in which (as for Heidegger) thinking and being are somehow elided. In this sense, the poetic imagination intuits rather than knows…”
From “George Oppen and the Poetics of Quotation” by Peter Nicholls
Editions Of You 
Okay…I have been away from any contact with the poetic scene for a long time and coming back into it, I find writer’s using other writer’s lines without overt attribution. They italicize, or set them apart, or place them in quotation marks, but do not necessarily name the original writer. What is the rule these days? Probably not a simple answer but you opened the door here, so I am hoping you have something.
margo
Hi, Margo! From what I’ve seen of poetic practice here in the Philippines, collections usually include an afterword of explanatory notes where quoted lines from the poems are pointed back to their sources. As for whether or not they are italicized, set in quotation marks or not, I guess that depends on what kind of effect you’re going for, like what Nicholls reads in Oppen.
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