Just about to take delivery of an Intex 15' diameter above ground pool -
one of the metal frame and liner type. As the garden slopes, I need to
create a level decking to sit the pool on.

The plan is to build a 16' x 16' deck from joists and bitumen coated OSB.
the pool will sit on this and round the pool decking planks will be laid
to give a finish to walk on. 95% of the deck will be under the pool.
The slope of the garden means one end will be at ground level, raising to
about 18" at the other end. The whole structure will be supported by
block pillars (apart from the the end at ground level which will sit on
the ground via concrete flags) and 4 cross joists.

An ASCII art diagram may help the picture (X are block pillars):

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X-|-|-|-|-X-|-|-|-|-X-|-|-|-|-X

The question is, to cope with the massive weight of water, should I use
6" x 2" joists at 12" centres, or will smaller timbers and/or larger
centres be okay - or a combination of sizes? Also, will a single layer of
16mm OSB me sufficient for the deck surface, or would this be better
thicker?

Access to the garden, and time are the main reasons why this has to be
built like this, rather than building retaining walls and filling with
soil.

Any comments and feedback would be great thanks.