Installing new osb on top of the old cedar is dumb. Spend the money and to a complete tear-off. Beyond that, long enough nails aren't your issue. The real issue is load calc for the existing rafters. Since you plan on using Arch shingles you should reduce the remaining dead load as much as possible. Arch shingles are much heavier than standard 15 or 20 year.

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Accurate Inspections & Consulting, Inc.
773/844-4AIC
Markus Keller