Oskar Werner did not receive an Oscar nomination for portraying Jules in Jules and Jim.
Jules and Jim is directed with a certain dreamy quality by Francois Truffaut. The music is always has this certain ease, as does the cinematography, and even the narration has this certain calm to it. There is a lightness kept to the story that stays even when one break in the story is a war. The earliest scenes of the film are shown all as some romantic vacation of sorts as Jules, and his friend Jim (Henri Serre) seems to live a life of such ease. This section in particular more of depicts images of the friendship between Jules and Jim, as well as Jules romance with Catherine, rather than detailed interactions between the two. Werner's performance is very much limited, though he is effective in depicting a certain spontaneous joy as we see him randomly live his life with Jim, and Catherine. Werner's work, like the film itself at this point, is purposefully breezy. as Werner quite plainly portrays a man enjoying life. This eventually ends on Jules marrying Catherine, and with that the film makes its first shift with both Jules and Jim entering into World War I.
World War I is only shown to be a brief distraction in terms of the story. The tone changes slightly, it still retains the same style, but Jules, Jim and Catherine all seem to slow down a bit. Jules's letter to Catherine is the moment of change for Werner's performance as he becomes far more restrained and frankly less enthusiastic in his performance. Werner is indeed quite effective in realizes the wear of the war, and the changed caused by just through his performance as there is clearly less joy in Jules, as he writes the letter to his wife. This continues as the story does as even after the war it becomes clear that Catherine's nature will cause her to ignore him, and in this case in favor of his friend Jim, even though the two have already had children. Julies, even though Catherine is leaving him, does not react as a jilted lover, and instead Werner depicts Jules as very much resigned in this fate. Werner, making use of that earlier scene during the war, actually makes this a fairly natural transition, and allows Jules's nonchalance towards Catherine's pursuit of Him, believable.
Werner carries the right sort of melancholy quality to Jules in any scene in which he discusses Catherine with Jim, as Werner echoes a certain despair, but as much depicts a sad understanding of exactly the woman she is. This leaves Jules in a strange spot, as well as Werner in a rather difficult place within the film as well. Jules actually becomes quite static for basically the second half of the film, as Jules must merely observe Jim and Catherine's behavior, while only really staying around because he's the father of Catherine's children. To Werner's credit though he actually isn't overshadowed, despite the story forcing him into such a position. Werner manages to get by through extremely low key charm, and just as well keeping a consistent honesty in his portrayal of Jules as he just keeps to himself while Jim and Catherine live out their problematic affair. Though Jules is often ignored as that affair becomes more intense, the final moments of the film end up from Jules's perspective. Even in this it is somewhat limited, though Werner's silent final moments are fairly moving as he reflects Jules's loss, as Jules once again remains the odd one out. Werner gives a good performance as his withdrawn yet engaging screen presence keeps Jules from being forgotten even when the film seems to have forgotten about him.