88 AA gun & opportunity fire

[Poor Yorek]

The 88mm was mostly a direct fire weapon, where does it say it cannot fire opportunity fire?

In PG, the 88mm has two fire types, BF and AT. Using AT fire, such a unit may make one OF attack per turn (which uses up its activation). The unit may not take OF using the BF value (see OF rules 13.0). The unit lacks a DF value with which to make OF attacks against units susceptible to DF.

BF = bombardment fire
AT = anti-tank fire
DF = direct fire
OF = opportunity fire