I happen to have the 1989 740 Volvo green electrical manual. There is no mention anywhere of the oil pressure gauge accessory. The gauge is likely intended for a separate cluster and wired independently of the instrument cluster wiring harnesses. BTW The wire harnesses for the Yasaki and VDO clusters are the same. I'm also not sure where the sensor is supposed to go in the block, perhaps on a Tee with the oil pressure warning light switch.

For the + and - wires you have, the + would go to a switched and fused source of +12V when the ignition is in KPII. Off the top of my head, the blue-yellow wire on the defroster switch would be a good source. The - would go to any convenient chassis ground, like the black wire on the defroster switch. There may well be a separate connection for dash lighting (which you can connect at the dimmer switch) as well as the wire going to the oil pressure sensor. If you don't have a turbo, you may be able to steal the turbo boost gauge wire going to a harness connector in the engine compartment.

If you're putting it in the instrument custer and want to attach to the wiring harness there, here a few usful connection points I can spot. In the 12-pin C connector (upper right on the back of the cluster) there are some possibly free wires to a connector in the engine compartment, a red/white wire on pin 2, if present, is for the turbo boost gauge and a white/black wire on pin 9 of that connector, if present, is for the washer fluid level sensor. A green/black wire on pin 11 of that same connector goes to chassis ground. In the 8-pin A connector (lower left of back of cluster), the blue wire on pin 1 is dash lighting (a variable ground). I'm having trouble spotting a suitable +12V source from the ignition switch in the KPII position, possibly the green/red wire on pin 5 of the 7-pin B connector (upper left on back) -use a meter to see if that's a +12V source in position KPII only. Another possibly switched source of +12V is the blue/red wire on pin 4 of the 4-pin D connector (lower mid cluster) -again, please check with a meter. There's also a very interesting terminal H listed as a "+ supply", which on the Yasaki cluster is the rightmost lower tab terminal in the speedometer area of the back of the cluster. There's no indication of the wire color there (there may be nothing connected) and I'm having great difficulty tracing it off the cluster other than through all the warning light circuits, such as the oil pressure warning light, so that's something I would definitely look at with a meter as a good +12V source.

EDIT I see mention of a gauge for 760s and a writeup about putting it in a 740 here on Volvo FORUMS.

Please post back when you've figured it out so that others may know in the future.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now