The medrash[1] points out that while all of Klal Yisroel were busy fulfilling the hora'as shoah of asking for gifts from their neighbors (bezos Mitzrayim), Moshe Rabbeinu busied himself with taking care of one of the mitzvos (which is binding through all future generations), namely, keeping one's oath. The Torah tells us that before Yosef died he had his brothers and all his relatives swear to him that they would have their children swear that they in turn would have their children swear etc. that when the time for geula comes, they would take his bones with them to be buried in Eretz Yisroel[2]. Yosef knew that the return to Eretz Yisroel would not take place during the lifetime of his brothers; they all knew from the prophecy of Avraham Avinu that the galus would last for four hundred years. Therefore Yosef didn't have the brothers swear that they would take care of it, but rather had them swear to have the next generation swear etc. that it would be taken care of when the time came.
In Shir Hashirim we read about the three oaths taken by the Jewish nation many centuries ago[3]. No one alive today remembers ever taking these oaths. Apparently it wasn't necessary for each generation to have the next generation accept these oaths. They were accepted at one point is history by the Jewish nation as a whole, and automatically all future generations are bound by these oaths, similar to a treaty entered into between two nations, which is binding on all future generations, since they too are a continuation of the original two countries. Based on this point, the Ragachover Gaon[4] raised the question, why was it necessary for Yosef to have his brothers swear that they would have the next generation swear etc.? Why didn't he simply have the brothers swear representing Klal Yisroel, and that shavua would automatically be binding on all future generations?
[The gemara[5] tells us that a minhag is binding miderabanan just as if one had accepted upon himself a neder, and we know that both an individual minhag tov as well as a minhag hakehilla are binding; so clearly there can be an individual neder as well as a neder or shavua of the kehilla.]
To this the Ragachover responds that before matan Torah there didn't yet exist a concept of a tzibbur or a kehilla of Klal Yisroel. The gemara[6] tells us that strictly speaking, the concept of a goy constituting a single entity only applies to the Jewish people. The other nations are really not considered a kahal, but rather a collection of many individuals[7]. When the Jewish people accepted the Torah, this unified us to create the concept of a tzibbur. To use the expression of Rav Saadia Gaon, "our nation only achieved its status as a nation through its Torah."
[When we bensch Rosh Chodesh the minhag is that the chazzan holds on to a sefer Torah and declares, "chaveirim kol Yisroel." Rav Soloveitchik pointed out that from the Rambam it would appear that nowadays that we have no Sanhedrin, the responsibility of establishing the Jewish calendar by declaring when Rosh Chodesh will occur is given back to the Jewish people. The chazzan holds the sefer Torah to demonstrate that this is what binds us and unites us to become one goy, and thereby enables us to determine the Jewish calendar.]
Yosef died many years before ma'amad Har Sinai, at a time when a Jewish nation as such did not yet exist. Therefore he couldn't have had the brothers take an oath as representing Klal Yisroel, rather their oath was an individual oath (shavu'as hayachid.) Since Yosef knew that they would not live to see the geula, he had them swear that they would have the next generation swear etc. to take care of his burial. But Shlomo Hamelech, who lived after mattan Torah, wrote in Shir Hashirim about the shavu'ah taken by Klal Yisroel that is binding on all future generations.
[1] Yalkut Shimoni to Mishlei(10:8) #946
[2] Rashi to Parshas Beshalach (13:19) quoting from the Mechilta
[3] See Kesubos 111a
[4] Sh'eilos U'Teshuvos Tzofnas Paneach, New York 1954 (#143,2). Also see Beis Yitzchok volume 39 page 513
[5] Nedarim 15a
[6] Nazir 61b
[7] Rav Soloveitchik was fond of quoting from the Radak that the word goy comes from the word geviyah which means "a body"; the implication being that all the members of the nation of Klal Yisroel are considered as if they join together to constitute "one body". Various gedolim were careful to recite as one of the morning berachos, "shelo asani nochri" instead of, "shelo asani goy", because the berachos and the tefillos are supposed to be recited in Biblical Hebrew, and in Biblical Hebrew we, the Jewish people are the only goy! (See Eretz Hatzvi pages 118-120; Nefesh Horav page 107, #1.)