Oba used guns to great efficency against the Takeda clan but which war lord (or faction) used them for the first time? I seem to recall that some of the shoei order had access to firearms and that is where Oda got the idea to use them en mass but I cannot find the source.
Around 1542 (the date is not certain) a chinese junk was blown onto the shores of the island of Tanegashima of the southern coast of Kyushu. On board were three Portuguese travellers, the first Westerners to land on Japanese soil.
To Lord Tokitada, the daimyo of Tanegashima, the most strking thing about the stranded Westerners were the guns that two of them carried. Having witnessed one of the Portuguese shoot a duck, he arranged for lessons and bought both guns for 2000 taels of gold.
Tokitada employed his swordsmith, Yatsuita Kinbei, to make copies of the weapons, going so far as to trade his seventeen year old daughter for the technology to cut a screw for the breach plug.
Within ten years the weapons were being manufactured on a much larger scale and began to appear on the battlefield. I don't have a reference for the first time they were used on mass, but two early adopters that have stood out:
If I find a reference with a more specific instance, I will update it here.