Firstly for best results, I would recommend waiting for quite a bit more than a week after installation before oiling, In Israel I used to wait at least 4-6 weeks before sanding and oiling my decks.
My reasoning being that freshly installed decking still has a high natural oil content and its mill glazed surface. Applying oil to the deck at such an early stage makes the wood look great but as oils dont mix very well the effect is largely cosmetic and short lived. Leaving the deck for some time to climatise however allows those natural oils within the wood to dry out and drain from the surface making the deck more ready to accept a coat of oil and actually nourish the wood.
As for coating all sides you’re quite right, it can’t be done after installation. If oiling all surfaces is specified then you’ll have to coat it before or during installation(Exterpark can be supplied pre coated with oil). In practice though you needn’t do so, I’d say its far more important to ensure proper ventilation to the underside of the deck keeping the timber dry to prevent damage from damp. Let the oil on the upper surfaces protect the timber and prevent damage from the sun.