Step one - take this to the cider forum.
Homebrewtalk Cider Forum
Add sugar, or not: do you want a lower or higher alcohol content end result? Adding sugar basically just cranks up the drunk-o-meter - does not improve the taste, IMHO.
pH - buy pH test strips at drug store or brew shop. No real need, but you can.
O.D. - Perhaps you mean O.G., as in original gravity, a measure of the sugar content, more or less, and an indication (along with F.G. - final gravity) of the resultant alcohol content. Use a hydrometer to measure these.
Plus you are using juice straight from the apples, I think this is yet another reason to go for a simple cider.
but surely if I just leave it for ages it will all ferment off
(someone else feel free to jump in if I'm a little off... I'm little more than an educated noob myself)
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