Jackson Arnold is Brent Venables’ guy under center this season. However, some have questioned Oklahoma‘s ceiling with him as the Sooners’ starting quarterback. Seeing time in seven games last season while making one start in a losing effort in OU’s bowl game, the former five-star recruit has honed in on his skills offseason, according to his head coach. “I’ve probably exposed to more negativity on why we can’t accomplish certain things this year because of Jackson and the bowl game and things like that,” Venables said during an appearance on SEC Now. “And I just look at the guys around him. Th…