Bokken disappears after you triggered the fight with him to finish his quest.
There's a way to circumvent this bug: wait with turning the quest in until Octavia can cast Dominate Person. Trigger the fight, don't attack but dominate him. This puts him out of combat, talk to him. Then wait until the dominate wears of, he'll be hostile again, put him down and then talk to him again.
When you leave Oleg's and then return you can see he didn't disappear and he'll also continue to bring stuff to the capital. I did this today, it does indeed work.
There's a way to circumvent this bug: wait with turning the quest in until Octavia can cast Dominate Person. Trigger the fight, don't attack but dominate him. This puts him out of combat, talk to him. Then wait until the dominate wears of, he'll be hostile again, put him down and then talk to him again.
When you leave Oleg's and then return you can see he didn't disappear and he'll also continue to bring stuff to the capital. I did this today, it does indeed work.