'о' is used before words that begin with consonants, including j-sound (for example: о еде, о единстве, о его etc.) 'об' is used before vowels. Exeptions: об пол, об руку, об стену. 'обо' is used before всё, всю, все, всех, что, мне, всём, всей, всех.

All of them mean "about". If a word starts with a consonant sound, you should use the preposition "о" before it. The preposition "oб" goes before the words which start with a vowel sound and some set phrases, e.g. "как рыба об лёд" The preposition "обо" is used before some words which start with consonants e.g."обо мне, обо всём, обо всех , обо что-то". You should memorise them.

о тебе о нём о нас о вас обо мне об этом предмете об этом человеке об этом самолёте ...

An addition to Denis' answer. "Об стену", "об руку" etc.... _are_not_ exceptions. It's just the different meaning... . It's 'об' + accusative case. Cf. "рука об руку" (hand in hand) "удариться об стену". Usually (obviously, 'hand in hand' is a different meaning! But English 'against' translates this "об" as well) it used with verbs describing deformations/damage/pains one/something takes moving into the static obstacle. Or the collision itself, when the damage was possible, but never actually happened. Or in the case, when collision is just somehow _felt_ (no damage at all) by the moving (even unanimated:) object (but not the static obstacle). Ударить(ся), пораниться, порезаться (об острый край), стукнуть, разбить (тарелку) etc. Sometimes it may be used with the verbs synonymous with 'throw'. - to throw against. The difference between throwing об стену and в стену is that In the case of об you think about the 'feelings' of the moving object at the moment of collision, while hearing 'в' you think about the direction and/or the point of collision.