Why is there a tilde with "oír"?

Question

Thanks a lot.

Answer 1

If there were not a tilde there then it would sound differently. A, E, O are strong vowels in Spanish, while I and U are weak.

If a weak vowel and strong one or two weak ones are next to each other then they form a diphthong.

Hacia, miedo, ruina, huele, paisaje

If the word is not meant to be a diphthong, a tilde is placed on the weak one to break up the diphthong.

Oír, Tenía, país, reír, río

If the tilde were not thereon oír, it would not sound the same.

Answer 2

link

Otherwise the oi would be a diphthong with the stress on the o.