Kokiri Tunic (いつもの服 Itsumo no Fuku?) is an item from The Legend of Zelda: Ocarina of Time. The initial tunic that Link is wearing at the start of the game, it is the traditional clothing of the Kokiri. The Kokiri Tunic does not possess any special abilities; nonetheless, it is the only tunic the Like Likes cannot swallow, and the only tunic that can be worn by young Link.
When Link changes into an adult after retrieving the Master Sword, the Kokiri Tunic is shown to have grown in size to accommodate that of Adult Link's, the explanation for which is never given. Interestingly, a white jumpsuit, which are worn underneath the tunic, and a pair of leather gauntlets have also been added to the adult Link's tunic. One possible explanation is that these alterations to Link's Kokiri Tunic were done by the Sage Rauru via magic while Link slept.
After being returned to the past by Zelda at the end of Ocarina of Time, young Link continues to wear his original Kokiri tunic throughout the events that follow including the events of The Legend of Zelda: Majora's Mask.
Although the Kokiri Tunic resembles the clothes Link wears in previous and later games, they are not referred to as Kokiri Tunics, presumably because they aren't Kokiri based. The same tunic worn by Link in Ocarina of Time is given to the Link in The Legend of Zelda: Twilight Princess by the Light Spirit, Faron; however, it is referred to as the Hero's Clothes.
It is likely that the Hero's Clothes are different from the Kokiri Tunic as aside from the fact that the Hero's Clothes incorporates what seems to be chainmail and a vambrace, providing improved protection, the Hero's clothes fit a teenage Link, which the Kokiri Tunic (that Link wore in Majora's Mask) wouldn't considering how it's for pre-teens in size. However it is possible that they were magically altered by the Light Spirits or the Hero of Time had them tailored to fit him as he aged, before switching to the armor that he wears as the Hero's Shade.
Another possibility is that the Kokiri Tunic and Kokiri Boots are magical in nature (which may apply to all Kokiri Tunics or just the one worn by Link). Given that Link is Hylian child and would eventually grow up into an adult, it is possible that the Great Deku Tree had his clothing either created with magic or magically altered them to be able to change as Link aged, to prevent him from outgrowing them as he grew up into an adult. This ability to change may also explain how Link's clothes seem to change with him when he transforms using Masks in Majora's Mask.
One of Link's DLC costumes is his Hero of Time incarnation from Ocarina of Time which is a part of the "Ocarina of Time Costume Set". This costume skin is referred to as Kokiri Tunic (referencing the fact that it was the Hero of Time's default tunic in Ocarina of Time) in the game's costume selection menu as well as the game's gallery. Young Link also wears the Kokiri Tunic and Kokiri Boots as his default outfit.
Link's Kokiri Tunic (Era of the Hero of Time Costume) can be obtained in Adventure Mode. It also appears as Young Link's default costume (Young Link can be also be unlocked in Adventure Mode as well).