I am going to guess (but I don't know off hand) that usually osioł takes -u, but when treating Osioł as your name -owi is used.
yes, and osiol took notes of the other thread, as he mentions in his previous post.
Most monosyllabic words tend to take -u (e.g. brat -> bratu)
I wouldn't hurry with such conclusions, have you analized more nouns? I never thought that the length of a word would influence its grammatical behaviour (in Polish).
In English we rely on prepositions to tell us who does what to whom, in Polish you don't always need a preposition:
This applies especially to Dative and Instrumental cases, when Dative is used for a recipient of an action and Instrumental for an means (a tool/intrument/vehicle) of an action.
Przyjechał/Przyszedł z osłem = He came with a donkey (so the animal was accompanying him, not used to transport him)
Przyjechał osłem (although it's better to say "na ośle")= He came/rode on a donkey.
Przyjechał do mnie nowym samochodem (He came to me in his new car).
Przyjechał do mnie z
nowym samochodem, który się już popsuł. (He came to me with his new car that was already broken.) - Here we don't know how he arrived, if he used his new car that was broken partially, but still functionnal or if he arrived in another car/truck towing his new broken car. if you add the preposition "z" to the noun in Instrumental it changes the sentence meaning (the stress is on the fact that he brought the car for a repair, not how he arrived).