Hey Wassily, let me know if I've got this right:
Your father's father was named Jim Douglas, who was in Marshall in the 1940s, but you're not sure where he came from or where he went. You're not sure who your father's mother was, but it may have been Okalena Riley. You also don't know where your father was born, but he may have been born out-of-state. He had some siblings but they are dead now.
Even though you're trying to find out more about your father's father, it would help to know info about where/when your father was born, & anything you know about his siblings even if they're dead now, because those pieces of information might help to find out more about your grandfather. For example, you might be able to get records about their deaths that also tells things about their parentage.
Can you provide more info?