"He is my son."
Translation:O benim oğlum.
There are a few words in Turkish that loose their second vowel when you add a suffix that begins with a vowel to them. "oğul" is one example of this. Many, but not all, of these are Arabic loan words.
Some other exmples are: şehir/şehri, ağız/ağzı, gönül/gönlü
Remember, if you add a suffix that begins with a consonant, the second vowel is kept: şehir/şehirde