Oh that one might plead for a man - A more correct rendering of this would be, "Oh that it might be for a man to contend with God;" that is, in a judicial controversy. It is the expression of an earnest desire to carry his cause at once before God, and to be permitted to argue it there. This desire Job had often expressed; see Job 13:3, note; Job 13:18-22, notes. On the grammatical construction of the passage, see Rosenmuller.
As a man pleadeth for his neighbour - Hebrew "the son of man;" that is, the offspring of man. Or, rather, as a man contendeth with his neighbor; as one man may carry on a cause with another. He desired to carry his cause directly before God, and to be permitted to argue the case with him, as one is permitted to maintain an argument with a man; see the notes at Job 13:20-21.

O that one might plead - Let me only have liberty to plead with God, as a man hath with his fellow.

O that one might plead for a man with God, as a man (x) [pleadeth] for his neighbour!
(x) Thus by his great torments he is carried away, and breaks out into passions, and speaks unadvisedly, as though God would intreat man more gently, seeing he has only a short time here to live.

Oh that one might plead for a man with God,.... That is, that one might be appointed and allowed to plead with God on his account; or that he be admitted to plead with God for himself; or however, that there might be a hearing of his case before God, and that he would decide the thing in controversy between him and his friends, when he doubted not but it would be given on his side:
as a man pleadeth for his neighbour; using great freedom, and powerful arguments, and having no dread of the judge, nor fear of carrying the cause for his neighbour; so Job wishes, that either one for him, or he himself, might be freed from the dread of the divine Majesty, and might be suffered to speak as freely to his case as a counsellor at the bar does for his client. The words will admit of a more evangelic sense by observing that God, to whom Job says his eye poured out tears, at the close of Job 16:20, is to be understood of the second Person in the Godhead, Jehovah, the Son of God, the Messiah; and then read these words that follow thus, "and he will plead for a man with God, and the Son of man for his friend"; which last clause perhaps may be better rendered, "even the Son of man", &c. and so they are expressive of Job's faith, that though his friends despised him, yet he to whom he poured out his tears, and committed his case, would plead his cause with God for him, and thoroughly plead it, when he should be acquitted. The appellation, "the Son of man", is a well known name for the Messiah in the New Testament, and is not altogether unknown in the Old, see Psalm 80:17; and one part of his work and office is to be an advocate with the Father for his friends, whom he makes, reckons, and uses as such, even all the Father has given him, and he has redeemed by his blood; for these he pleads his blood, righteousness, and sacrifice, to the satisfaction of the law, and justice of God, and against Satan, and all enemies whatever, and for every blessing they want; and for which work he is abundantly fit, because of the dignity of his person, his nearness to God his Father, and the interest he has in him. Gussetius (l) goes this way, and observes that this sense has not been taken notice of by interpreters, which he seems to wonder at; whereas our English annotator on the place had it long ago, and Mr. Caryll after him, though disapproved of by some modern interpreters.
(l) Ebr. Comment. p. 320, 321.

one--rather, "He" (God). "Oh, that He would plead for a man (namely, me) against God." Job quaintly says, "God must support me against God; for He makes me to suffer, and He alone knows me to be innocent" [UMBREIT]. So God helped Jacob in wrestling against Himself (compare Job 23:6; Genesis 32:25). God in Jesus Christ does plead with God for man (Romans 8:26-27).
as a man--literally, "the Son of man." A prefiguring of the advocacy of Jesus Christ--a boon longed for by Job (Job 9:33), though the spiritual pregnancy of his own words, designed for all ages, was but little understood by him (Psalm 80:17).
for his neighbour--Hebrew, "friend." Job himself (Job 42:8) pleaded as intercessor for his "friends," though "his scorners" (Job 16:20); so Jesus Christ the Son of man (Luke 23:34); "for friends" (John 15:13-15).