Learn Finnish: Minä kirjoitin otsikon.

This is consonant gradation in action. The verb kirjoittaa (to write) has a strong double tt. When we conjugate it by adding the past tense marker -i- and the personal ending -n, the final syllable is "closed" by a consonant. This triggers consonant gradation, changing the strong tt into a weak single t.

Why does otsikko (headline) become otsikon with one 'k'?

Just like the verb in this sentence, the noun otsikko undergoes consonant gradation. Adding the -n ending closes the last syllable, which forces the strong double kk to weaken into a single k.

Why does the object otsikon end in -n here?

Because the action was completed. When you finish a single, complete action in Finnish, the object takes this -n ending (often called the genitive-accusative). It shows that you finished writing the entire headline. If you were still in the process of writing it, you would use the partitive case (Minä kirjoitin otsikkoa).

Do I need to include the word minä?