What does that wire attach to? The oil pressure sender wire should be yellow with a red stripe. What colour is that wire before that crimp connector? Does it have the red stripe?
Even if the sensor's missing, there has to be something there, because every rear plate Mazda has made for decades has had an oil pressure reading point there!

 

before the crimp connector it just a black insulated wire. on the other end it clips on to a cap that goes over a small threaded bolt thing that comes out from the engine. theres a flat area next to it, that looks like it mayve been filled in with something, its covered with that yellowish looking stuff you can kind of see in the picture. nothing at all is the the size or shape of the sensor in the pictures.

 

I can see yellow between the crimp and the black sheath. It might be the oil pressure sender wire. Can you remove it and take a photo? That might be the temp gauge sender, which has a yellow wire with a white stripe.

 

Whats the object in red dots. Jpg attached. Nah. Thats not it. Some squid buggered the threads up on the sending unit and epoxied over the oil pressure hole. Thats the answer. If you measure aft of the water temp sensor hole approx one inch, thats where the senders hole used to be.

 

I think that's it...

 

I reflected on my answer after it was sent and decided other. I think we're on to a classic here. Someone, thing, epoxied over the senders threaded hole. Off to sleep on it.

 

I'm at work so I cant check now, but im pretty sure the thing in the red dots is a hose. In that yellowish area theres a circular ident, so I guess it that means it was epoxied over. Now, is an oil pressure sensor valuable enough to try and put one in?