24 This is the disciple who is testifying to these things and has written them, and we know that his testimony is true. 25 But there are also many other things that Jesus did; if every one of them were written down, I suppose that the world itself could not contain the books that would be written. (John 21:24-25 NRSV)
Andreas J. Köstenberger, ‘‘I Suppose’ (oimai): The Conclusion of John’s Gospel in Its Literary and Historical Context,’ in Williams, P.J., Andre D. Clarke, Peter M. Head, David Instone-Brwer (eds.), The New Testament in its First Century Setting: Essays on Context and Background in Honour of B. W. Winter on His 65th Birthday (Grand Rapids, Michigan: Eerdmans, 2004): 72-88.
In this article Köstenberger takes a fresh look at the term oimai (‘I suppose’; John 21:25) in the context of debate over the authorship of the gospel; specifically, how the term oimai needs to be given due weight in the debate concerning the status of vv.24-25 as an original part of the gospel or a later addition(s) by a different author(s). After briefly surveying the various interpretations of the first person plural in v24 (“we know”) and first person singular in v25 (“I suppose”) as they pertain to judgements on authorship, Köstenberger turns to extrabiblical usage of the term. He offers a sample of instances from Greek literature from the first centuries BC and AD (Diodorus of Sicily; Dionysius of Halicarnassus; Josephus; Plutarch), and summarises his findings as follows (87):
the term oimai is a literary term frequently used by historians reflecting authorial modesty in stating a claim or opinion. It is commonly part of authorial discourse, not infrequently at the beginning or conclusion of a literary unit or at points of transition. In its extrabiblical instances the term regularly forms an inextricable part of the author’s argument that cannot be easily separated from the larger context by source or redaction-critical means.
Köstenberger interprets John 21:25 in line with these as an instance of ‘authorial modesty’ which constitutes ‘an integral part of the authorial message.’ (87) It is on the basis of a lack of precedent in his survey of an instance where a later editor/group of editors use the term to authenticate ‘the message of an original author or witness,’ that he concludes that its presence in John 21:25 ‘would render redaction-critical proposals unlikely’ (87).
This line of reasoning may fault on the grounds that it disallows innovation. However, I do think that the first person singular oimai (“I suppose”) needs to be given due weight and not interpreted against its grain (and without known historical precedent) to bring it into line with hypotheses of later aditions and communal authorship/redaction.
Köstenberger also mentions an article by H. M. Jackson in which he argues that the third person singular (“this is the disciple”) and first person plural (“we know”) in v.24 reflect the ancient conventions of self-reference, and therefore can no longer be seen as compelling grounds indicating a change of authorship in 21:24-25 (73-4).
 Köstenberger’s survey was limited to occurrences of the present-tense first person singular where it is followed by an accusative and infinitive, as in John 21:25.
 Köstenberger both states that he is building upon Jackson’s findings even while his proposal stands apart from it. H. M. Jackson, ‘Ancient Self-referential Conventions and Their Implications for the Authorship and Integrity of the Gospel of John,’ JTS 50 (1999): 1-34.
*In the title and body of the article oimai is in greek characters. Unfortunately I do not know how to display greek fonts in WordPress, but if anyone does please let me know how*