Eleven O’clock
اَلسَّاعَةُ الْحَادِيَةَ عَشْرَة

Twelve O’clock
اَلسَّاعَةُ الثَّانِيَةَ عَشْرَة

Note :
You need to use the definite prefix article "ال" with the ordinals because you’re referring to a specific hour and not just any hour.

اَلسَّاعَةُ ( the hour [o’clock])

دَقِيقَةٌ ( minute)

دَقَائِقٌ ( minutes)

نِصْفٌ ( half [to indicate 30 minutes])

رُبُعٌ (a quarter [to indicate 15 minutes])

ثُلُثٌ ( a third [to indicate 20 minutes])

صَبَاحاً ( in the morning)