To upgrade an Oracle, I'd recommend a Vortex Red Dot. Great quality for the price ($120-140 Strikefire, $140-180 Sparc). A number of places from PSA to Cabelas often have deals on them. Bushnell's $100 TRS-series is also a good option.
The other additions (for me) would be ammo, mags, and then maybe Magpul foregrip and stock...in that order. Maybe a basic sling and rifle case, if you feel inclined. Flip sights would be nice too, but that's not real practical without a handguard upgrade. And I wouldn't make that much of a project/investment out of it. Keep it simple. Ammo, mags, red dot, basic furniture upgrade.