The same difference between "I" and "me".
Why do you say: "I love my dog" but when you reverse it, it becomes "My dog loves me"? Why not "My dog loves I"?
That's something called "the object" of a sentence. In English, pronouns (I, you, he, she…) change according to whether they are the subject or the object of a given sentence. Turkish does the same thing: O → subject; Onu → object.
personal pronouns become: beni (me), seni (thee), onu (him/her/it), bizi (us), sizi (you/ye), onları (them)
So that's 6 words to keep in mind.
For the rest of the words, there is a clear-cut rule that you can find here:
This whole thing is called "the accusative case". It's a fancy word that refers to the object of a sentence.
Yes, but ‘o’ is even further away. I can't tell from here on the mobile app what other languages you might be familiar with, but if you know Spanish, then ‘bu’/‘şu’/‘o’ translates to ‘esto’/‘eso’/‘aquel’. If you add the element of location (English ‘here’/‘there’), then you get another triplet in Spanish, ‘aquí’/‘ahí’/‘allí’, which has the benefit that it can also be translated into German (in case you know that better): ‘hier’/‘da’/‘dort’. (I haven't learnt the Turkish for these yet, although the Internet suggests ‘burada’/‘şurada’/‘orada’.)