Norwegian would often follow a S-V-O pattern, but not always. The only thing that is certain is that in indicative mood, the verb will be in the second position. In this sentence, the time adverbial 'om sommeren' is first, so 'spiser' is second. After the verb, the subject should come in such sentences.
Firstly, you can't say "i sommeren". You can say "i sommer", which is a little hard to translate, but you can say it means "in summer" (implied this summer).
I think of "Om sommeren" is more general, and "I sommer" is more specific. I'll give you some examples:
"Om sommeren spiser jeg is" (In the summer I eat ice cream) "Om sommeren er det varmt" (In the summer it is warm) "Vi pleier å være på hytta om sommeren" (We are usually on our cabin in the summer"
"I sommer skal jeg til Polen" (In summer I'm going to Poland) "Lisa kommer hit i sommer" (Lisa is coming here in summer) "Hva skal du i sommer?" (What are you doing in summer?"
Hope this made it easier to understand!