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Buy o-rings as soon as you buy yor camera housing. Otherwise, in about a year, you will regret it (whan said o-rings prove hard to find, and get on time for your next diving tip).
As soon as you get them, lubricate them with silicon oil and stuff them back in the bag they came in, until needed in about a year or so.
Is a shame no vendor offers a combo of housing + extra o-rings right of the bat.