Oskar Werner did not receive an Oscar nomination for portraying Corporal Karl Maurer aka Happy in Decision Before Dawn.
Oskar Werner plays the eventual lead of the film as Happy, who we first see as a German medic and POW but later becomes a spy for the Allies. The film takes a bit of time as it illustrates the plot by spending time with the Americans, planning the mission and setting up another German spy, who has the most predictable character arc one could imagine. The film eventually finds its way to focusing on Happy, who happens to also be played by the best actor in the film. Werner even kind of steals the film before he even gains the stronger focus as he proves his ability onscreen, just through his eyes as he is able to express the quiet outrage in Happy as he decides to work against his home country, in part due to seeing his fellow soldiers despicable behavior even while detained. Thankfully the film restricts its focus upon Happy sooner than later, and we are given one of Werner's first English language performances. Now he might not be as assured as his work in The Spy Who Came in From the Cold, but that's a tall order to fulfill.
Werner once again has this real low key charm about him, and he immediately endearing in his portrayal of Happy. He importantly doesn't even need to try, there is just an innate honesty Werner brings to his performance, which is interesting given that he technically playing a traitor. Werner though brings basically home the message of the good traitor in his performance by bringing this effortless goodness that he exudes as Happy. This is also essential in that it makes it particularly easy to invest in Werner as Happy is sent back into his home country in order to discover an important bit of information. Werner actually has a particularly difficult challenge in that he really doesn't have anyone to work against in terms of portraying the man's on the mission, since Happy does not meet up with his liasons until near the film's ending. The rest of the time it is solely upon Werner to realize the struggle in Happy as he goes about his mission. Werner succeeds in this as he creates the sense of the underlying fear in Happy throughout the scenes, but does even more than that.
Werner gives further understanding of Happy through very nuanced indirect reactions within other interactions. For example there is great moment where Happy learns that his father is nearby, and Werner is able to express the concern in Happy for him while still keeping the shell of a soldier just going about his duty. There is so much dependent on Werner to capture so much of the emotional weight of the story. Werner never is lacking in this though and adds so much substance to the side relationships Happy strikes up while on his mission. This includes two "lowly" sorts one a woman few others care about and an affable fellow soldier. In both Werner presents such a palatable empathy in Happy and in turn makes those character more meaningful than they would have been otherwise. Werner is especially moving in a scene where he tries to passionate save someone from death. This all while Werner never loses the struggle of the mission in his very being. He keeps that pivotal central tension but finds the right amount of substance that benefits the film greatly. Eventually the film ends on a straight escape scene where technically some of that substance found earlier seems lost. Werner though proves his worth one last time though in the escape when Happy sacrifices himself for the sake of the mission. Werner has made Happy such a likable character that when this happens it is rather heartbreaking, even if the film itself still doesn't seem like it quite is appreciating what Werner is doing for it. It's performance which elevates the film, and though it might not be as assured as his later work, it is a strong early indication of Werner's talent.