I doubt you'll find any specific reference to OPI-on-a-try in the rulebook, but it hardly matters.
Think about it: If you have OPI during a try, the following scenarios can occur (assuming NFHS ruleset):
1) OPI, try is converted.
2a) OPI, try is not converted. Team B accepts the penalty.
2b) OPI, try is not converted, Team B declines the penalty.
In #1, Team B will obviously accept the penalty, 15 yards + loss of down, thus ending the try.
In #2a, Team B accepts the penalty, 15 yards + loss of down, thus ending the try.
In #2B, Team B declines the penalty, thus ending the try.
Notice that #2a and #2b are, for all intents and purposes, identical.
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