It is possible to use items on Minish Portals (tree stumps, pots, stones, dungeon portals). All items except for the Gust Jar and the Roc's Cape work.
*For some items only specific sides work, for example, to use the Ocarina on a portal, you have to perform Portal Items from above a portal. For most items, however, the side you perform Portal Items on doesn't matter.
Depending on the items you use, a few different things can happen.
For most of them, the item will be used once on the ground, and then a second time on the portal. Note that something in a bottle will not disappear until the second use, so you can technically turn one Fairy into two.
The Mole Mitts will put Link a bit more upwards on stones and dungeon portals due to the knockback. This can be used to jump onto a wall in Fortress of Winds.
Bonking on a Portal with the Pegasus Boots will push Link from the Portal, but some of the effects from being on a Portal don't disappear:
Using a bottle will allow Link to take damage on the portal. Dying will put Link into the same state as bonking with the Pegasus Boots.
Uses for Portal Items: Fortress of Winds Big Key Skip
Combining Portal Items with the Ocarina Glitch allows you to move on Minish Portals, which in turn allows you to jump over any solid object.
After getting the Ocarina Glitch and still being on the frozen screen, jumping onto a portal will (almost always) make Link jump at an angle, instead of jumping directly onto it. By doing Portal Items using the Ocarina or by just simply jumping onto the portal from above, Link will be far enough away from the Portal to not be able shrink by pressing R after cancelling the Ocarina Glitch. Using this, you can move around by using the Gust Jar on the Portal by pressing R+Gust Jar at the same time (works with every item). Since Link is still technically on the Portal, pressing any directions will make Link jump off of it. Usually, Link will not cover any distance by doing this, however, if there is a wall (or anything else solid) next to Link, he will jump over it. You can also increase the jump's range by a tiny bit by either taking damage or using the Roc's Cape once.
Increasing The Jump's Range
Moving on Portals can also be used to jump over loading zones. To do this, simply position yourself right next to a loading zone (you can't trigger it when youre moving on a portal) and press the direction towards it.
Jumping over a loading zone will put Link out of bounds, with most of the loading zone triggers from the original screen still intact. Since there is no collision in the out of bounds area, Link can reach some loading zones from unintended angles, allowing you to access out of bounds areas on other screens.
Interestingly, loading a screen from OoB and spawning in OoB will actually somewhat load the correct map and put Link on it, but will not load any actors and tiles such as grass will not disappear after slashing it (though fire will still get rid of grass). Despite the pixel vomit, you can still navigate Link to other loading zones as long as the camera still follows Link. Pausing the game will cause the camera to go back to the start of the map, which hinders you from accessing screens that aren't triggered through white loading screen transitions. Since portals don't work correctly on these screens, you are able to use them to jump over other loading zones once again, as shown in the video.
In terms of science, there is still a lot we don't understand about this glitch, so the videos also show things we aren't able to explain quite yet, but due to the fact that it can only be used on very few screens and takes a long time to set up, there will probably never be any uses for it.