I can't offer an opinion on the Oracle, as I was well past the early game by the time this GB was introduced.
to your second question, yes, I would certainly look to build a second (and shortly thereafter, a third) GB. Which one will depend on your style of play.
If you wish to build up your production of Goods and Supplies, then the Lighthouse of Alexandria would be my next GB, as it does both. If you're going the aggressive neighbor route, then a Statue of Zeus (bonuses to attack and defense of your attacking army) would be my next GB.
Exceptions: if you are part of a good Guild, you might be able to make arrangements to trade for the Goods necessary to build an Arc, which gives you a bonus to Forge Points, Medal and Blueprints acquired through donating to other player's Great Buildings.
The same arrangement can perhaps be made to build a Temple Of Relics, which allows you to participate in the Guild Expedition, which can yield very good rewards.
In fact, a similar arrangement can be made for any GB in the game, but I would suspect those two (along with Alcatraz, which gives unattached military units) would be the most popular choices for such arrangements.