No declaration required for OLC. You do need to close the triangle with
less than 1000m altitude loss.

The BHC (Baron Hilton Cup) needs to be declared but the 25/28% rules do not apply. I believe the tasks can have only one or 2 turnpoints.

For the triangle 'bonus' in OLC-Plus, the 25/28% rule does apply. A declaration is not required.

For FAI tasks the triangle needs to satisfy the 28% rule up to 750km, then 25%. A change in the OLC-Plus rules is the 28% goes up to 500km then 28% afterwards. So it is a bit easier to make a triangle on OLC.