Confining remarks to NYSE, the open is, in theory, the ........one.......... price point that cleans out the MOST open orders.
In reality it's in accordance with the specialist's inventory posture and agenda. A posture is what he has, an agenda, what he's going to do. He, the entity most married to the equity, either wants aquire or shed.
Gaps obviously create fast clean price movement FOR the specilaist, but not the outsider. These movements are often used with a news alibit (whether valid or fluff). IF you're holding, a gap UP can be considered a gift.
Gaps downs have more credence IF they remain above the 3 day midpoint.
Predictability? LOL.
About the best indication is a strong close suggests a strong open. Whether it holds is only clear in hindsight.
Statisitics indicate the open is THEE high or low for the day 17 to 23% of the time. Hence it's better to wait for an opening range to establish as reference points.
Index opens can be deceptive due to delayed opens of components as well as large stocks skewing it.
Lastly, the opening hour is refered to as amateur hour for a reason.