I can't get it...
I see! Thank you very much!
Tsumami - You eat some snacks with alcohol (beer or sake). This is Tsumami.
Oyatsu - between lunch and dinner (or if you feel hungry), you eat some snacks. This is Oyatsu. If you eat Oaytsu with beer, this will be Tsumami. I think.
With alcoholic beverage or without alcoholic beverage is a key difference.
And also, if you eat heavy food with alcoholic beverage, this is not Tsumami.
Tsumami should not be heavy food. My favorite Tsumami is "Takowasa."