When Avraham instructs his servant, assumed to be Eliezer, to find a wife for Yitzchak, he makes him swear in the name of Hashem, G-d of heaven and earth, that he will not take a wife for Yitzchak from the women of Canaan, but that he will go instead back to Avraham's birthplace and find a wife for Yitzchak there (בראשית כ"ד:ג,ד). Many have difficulty understanding why Avraham had to demand that Eliezer take such an oath. Why couldn't he just have given him his instructions, and trusted him to carry out the task properly? Wasn't he a loyal servant of his, as the previous Posuk (פרק ב`) indicates, stating that this servant was the one whom Avraham trusted to run his entire household?
The Ramban answers that Avraham feared that he would die before Eliezer got back. By having Eliezer take an oath, Avraham was assured that Yitzchak, his only son, would get a proper wife. Along the same lines, The Sforno answers that it was necessary to demand an oath from Eliezer as opposed to simply commanding him because Avraham feared that if he died Eliezer might be bribed to find an inappropriate wife for Yitzchak. However, once Eliezer had taken a sacred oath Avraham felt that he would never accept such a bribe. The Ohr Hachaim adds that Avraham's hidden intention in the oath was also to warn Eliezer not to marry off his own daughter to Yitzchak, something Eliezer apparently might have wanted to do as mentioned by Rashi later (פרק כ"ד פסוק ל"ט).
A second question that arises is why didn't Avraham direct these instructions to Yitzchak himself, since he was certainly old enough to attend to this matter himself? The Ramban answers that since Avraham himself was too old to go to his old country and family, and since he would not let Yitzchak leave the Holy Land, he directed his instructions toward Eliezer, the one who would actually go to the land and seek a wife for Yitzchak. Once he required Eliezer to take an oath on the matter, Avraham saw no need to command Yitzchak directly, because he knew that Yitzchak would understand how serious the issue was and would not disregard his father's wishes.