Oath of Fealty
The Oath of Fealty was a serious matter that was not entered into nor revoked lightly.
Oaths of Fealty took many forms and varied quite a lot throughout the Middle Ages.
The following is an example of an oath between a Duke and a Queen. This would be a more serious oath in which the Duke strongly supported the Queen; some Dukes could (and would) make themselves less obligated to a monarch. Obviously, the religious references and place names would also change depending on who was swearing fealty to whom.
CHARTER OF HOMAGE AND FEALTY
In the name of The Aenil, I, Septimus Scarlett, Duke of Lasop, in the presence of my son, Lucius, and of my squire, Bohemond Dodd, my fellow nobles, and of many other honorable men, who have come to this temple of the Magna Aenilia Ecclesia to honor the Crown of Ethiala; since Queen Kitiara Silvan, the present bearer of the Crown of this Realm, has asked me, in the presence of all those above mentioned, to acknowledge to Her the fealty and homage for the castles, manors, and places which the patrons, my ancestors, held from her and her predecessors as a fief, and which I ought to hold as they held, I have made to the Queen acknowledgment and homage as I ought to do.
Therefore, let all present and to come know that I the said Septimus Scarlett, Lord and Duke of Lasop, acknowledge verily to thee my lady Kitiara, by the grace of the AEnil, Queen of Ethiala, and to thy successors that I hold and ought to hold as a fief in Lasop the following: that is to say, the castle of Élasop; and the manors of de Blanc, of Kandurell and of von Bodenstein; for each and all of which I make homage and fealty with hands and with mouth to thee my said lady Queen Kitiara and to thy successors, and I swear upon mine honor and the Aenil that do guide it that I will always be a faithful vassal to thee and to thy successors in all things in which a vassal is required to be faithful to his lord, and I will defend thee, my lord, and all thy successors, and the Crown and all your men and their possessions against all malefactors and invaders, at my request and that of my successors at my own cost; and I will give to thee power over all the castles and manors above described, in peace and in war, whenever they shall be claimed by thee or by thy successors. Moreover I acknowledge that, as a recognition of the above fiefs, I and my successors ought to come to the Crown, at our own expense, as often as a new Monarch shall have been made, and there do homage and return to him the power over all the fiefs described above. And when the Queen shall mount her horse I and my heirs, Dukes of Lasop, and our successors ought to hold the stirrup for the honor of the dominion of Ethiala; and to Her and all who come with Her, to as many as two hundred beasts, we should make the Queen's purveyance in the city of Lasop proper, the first time She enters Lasop, with the best fish and meat and with eggs and cheese, honorably according to Her will, and pay the expense of shoeing of the horses, and for straw and fodder as the season shall require.
And if I or my sons or their successors do not observe to thee or to thy successors each and all the things declared above, and should come against these things, we wish that all the aforesaid fiefs should by that very fact be handed over to thee and to the said Crown of Ethiala and to thy successors.
(to which the liege replies)
I, therefore, the aforesaid Queen Kitiara Silvan, by the grace of the Aenil (or whatsoever Her Majesty shall hold true and dear), receive the homage and fealty for all the fiefs of castles and manors and places which are described above: in the way and with the agreements and understandings written above; And I promise to thee and thy heirs and successors, Dukes of Lasop, under the religion of my order, that I will be good and faithful lord concerning all those things described above.
Made in the (season) of the year (year), in the reign of Lasop. Seal of Septimus Scarlett, Duke of Lasop, High Marshal of Ethiala; seal of Bohemond Dodd; seal of Lucius Scarlett; seal of Her Majesty the Queen, who has accepted this acknowledgment of the homage of the said Duke.
You can always Google your own, however here are several links to pages that discuss or detail various oaths of the medieval age.