And have adapted it to my needs, to fit inside an old SLR camera.
I'm looking for a small form way of powering it to fit inside the camera case. Something of note is that the 5v pin is already in use to power the display so I guess I can't power the nano through it. There's also no space to use the usb port.
I've got space for triple As or a 9v square battery or a lithium ion.
More precisely: you should connect your 5V battery to the VIN pin of the Nano and to the VCC pin of your OLED. The 5V pin on a Nano is a 5V output pin of the on-board 5V DC regulator
see also https://technobyte.org/2016/07/power-up-the-arduino-uno/ (Uno similar to Nano)
Success, photoncatcher
photoncatcher:
you should connect your 5V battery to the VIN pin of the Nano and to the VCC pin of your OLED.
No, you would need at least 6.5V battery to connect to Vin pin. If your "battery" truly is 5V you should connect it to the 5V pin. For example 4xAA or 4xAAA NiMH rechargeable are probably near enough to 5V to connect like that. But not 4x non-rechargeable!
A 9V battery is not a good choice. Their capacity is low and most of that will be wasted by the regulator, because the more voltage that needs to be dropped, the more capacity is wasted.
A Li-ion battery might work with Nano if you are luckly. But it won't have enough voltage to guarantee that a Nano will run at 16MHz. With Li-ion, a 3.3V Arduino such as an 8MHz Pro Mini would be a safer choice.