Interestingly, you're actually thinking in the right direction! Turkish comma usage is a lot different than English comma usage. If a sentence is long and torturous with lots of modifiers that might get subject and object and main verbs confused you might potentially have a comma after o.
This sentence is too short for that by far though and the subject and object are pretty clear (çilek is even marked with the accusative). You COULD disambiguate the O from the çilek but it's not really super important to figure out sentence meaning and context is clear.
You would only need a comma to make the subject clear if it were something like "O, [long rambling dissertation packing multiple phrasal statements about agriculture and modern life into modifiers of the eaten strawberry extending this ridiculous sentence for something like six lines until you want to claw your eyes out because you can't even figure out the point of the sentence, what are we describing again] çileği yer." :P
Çilek is the nominative form; çileği is the accusative.
You can read about consonant mutations here: https://www.duolingo.com/skill/tr/Accusative/tips-and-notes.