Thanks for reporting back,

SunExpress should not have subclasses. Probably they don't use the XQ system at BSL but use the system of the ground handling company so that might show O as zero as the indication of no fare buckets being available.

For Turkey departures or international departures that uses the XQ system, the subclass would show as "E" for Economy Class on all boarding passes.
The X-Leg seats in front does only indicate extra legroom and does not indicate another class of travel as you mentioned.