Pokemon Omega Ruby

A lot of the X/Y tips also apply to Omega Ruby, if you want to scroll back and find those. "Delta Episode" isn't some separate content, it's just what the post-Elite 4 content is called. You won't be able to 100% clear out every area of possible species to catch until after the main story, when a bunch of foreign pokemon get mixed in, so don't kill yourself over trying to get platinum medals in every zone early on.

Speaking of which, DexNav is great. You can use it to chain encounters and eventually catch overpowered versions as well as scope out their level, held item, egg moves, IVs, and so on. If you've seen a pokemon but haven't caught it, its silhouette will show up; unseen pokemon will be a blank space but you'll know you're missing something by the space and by the bronze/silver "almost there!" medal in the corner.

Using the lower Map screen lets you keep track of trainers spoiling for another fight, berry patches, spots with uncompleted DexNav, and secret bases. You can fly to cities and routes you've discovered, which is very handy on some of the longer routes.

You should definitely establish your secret base early and fill it with a variety of trainer types. In addition to training against them, they each have 1-3 special tasks they can perform every day, like giving away free items, training your pokemon up a level, increasing their happiness, etc. On the other hand, you might want to fill your base with pre-fab trainers with lvl100 Audinos that kill themselves and give you loads of EXP whenever you like. Up to you.

Super Training is essentially EV training in a more palatable format. I don't know if you know how EVs work but I won't sperg out about it unless you'd like me to. The long and short of it is: you can train up a stat's EVs (effort values) up to 255 EV points per stat, and 510 EV points total per pokemon. EV points translate into actual stats at a 4:1 ratio. It's totally unnecessary to game the system with Super Training in a casual playthrough (you'll gain EVs naturally by fighting wild pokemon and eventually will be pretty well-rounded), but it is very important to eek out every possible advantage if you are playing against serious nerds online.

The actual Super Training minigame is pretty straightforward. Higher stats will let you perform better. If you find yourself losing a lot, level up a bit or come back in an evolved form. Green, Black, and Orange "shot"-havers should focus on hitting single targets as hard as possible, Blues and especially Yellows should clear out white targets as fast as possible and then punish the stationary orange one when it pops up. All of this sort of goes out the window when you're in the 200-300 stat range and taking down balloons in <10 seconds.

Ditch small bags and unneeded medium bags. Hoard Double-up, Reset, and Soothing bags. There are 4 or 5 bags that make training easier in different ways, which you should probably save for the Secret Training challenges so you can get the evolution stones you want.

Besides KOing certain legendaries, I don't remember there being anything permanently missable. You'll definitely want a guide to find all the post-game legendary pokemon since some have very specific conditions to showing up. Besides that, the game is particularly hand-holdy and will give you everything you need to succeed.

At the very end you will be fighting
Deoxys using your freshly caught M-Rayquaza
but you can switch in a different flying pokemon if you prefer. I'd recommend a maxed-out one with False Swipe, like a Fearow or something, to make catching it easier on you.