Jessy,
Glad you joined the forum. Thanks for coming to the rescue of Oscar.
Ditch the crickets and find some night crawlers.
Here's a link to the forum care sheet. It will answer a lot of questions for you.
Garter Snake Care Sheet - Caresheets

Provide a good water supply and some additional heat and you'll just have to wait and see if Oscar is going to survive his injuries.
It's great that you took the time to inter seed for him. Nice job