Over Heaven is achieved when you use Dio's Diary. When used, your stand will change its color scheme to yellow / gold, white, and a cream like color. It also gives you a stat boost. When you use Dio's Diary you cannot use the Requiem Arrow and vice versa.
Over Heaven unlocks an exclusive set of abilities, such as Hamon Mastery and Self Healing. Here is some info on the moves:
Over Heaven Blast: A projectile which it's damage increases with special.
Self Health: A move which allows the stand user to heal themselves, the move's effectiveness can be scaled by upgrading special.
Hamon Mastery: A passive skill which is obtained when you use Dio's diary after you obtain hamon with your stand, you have a 1/3 to collect this. Hamon Mastery allows you to negate 20% debuff and reduces the reduction of the hamon bar.
Sun Immunity: This is similar to Hamon mastery. It is a 1/3 chance to obtain, except it gives the stand user immunity to sunlight. It is a passive skill that is obtained when you use the diary on a stand whilst after you use the Stone mask, This gives you immunity to the sun, meaning you won't burn when it's day.
Time Stop - A move that works similarly to requiem Time Stop. The only differences being that Over Heaven time stop gives off a yellow coloring instead of inverted colors, and Over Heaven time stop gives 1 more second of stopped time than base Requiem time stop.
Reality Overwrite - a move similar to hard punch but with more power, if a stand user is hit his stand will withdraw
Over Heaven will stay white and gold, even if you use a color arrow.