Wait. Are you sure? The top line is the return line to the oil tank from the engine.
Anyway. Make sure the wires are snug on the oil pressure gauge and on the oil pressure sending unit. If anything was loose, that was probably the problem. Try it. If not, then proceed.
Pop the nose cone off the right side, take out the spark plugs, jack the rear wheel off the ground, rotate the engine until the timing marks on the gears and lock the outside chain tensioner with an Allen wrench. Carefully remove the gears trying not to move the cam shafts as you do. Then carefully remove the tension on the tensioner and remove the snap ring. Remove the outside tensioner.
Push up on the oil pressure relief valve spring and gently knock the roll pin out. Remove the spring and the plunger (I use a popsicle stick pushed up inside the plunger to gently pull it free). Clean the plunger with carb/choke cleaner to get any varnish off it. Clean the port in the plate very carefully to make sure it is VERY clean. Using the popsicle stick trick, dip the plunger in clean oil and make sure it moves freely and easily in the port in the plate. Reassemble the oil pressure relief system first making sure to use clean oil on the plunger when installing that. Make sure the spring is completely above the roll pin when you install the pin.
Reassemble everything. If the cam shafts have moved, then just realign all the timing marks. Make sure you have the marks on the cam shafts themselves aligned with each other.
When every is buttoned up, but before installing the plugs, roll the engine over by rotating the rear wheel with the tranny in 5th or 6th. Listen for unexpected sounds. Then roll the engine over with the starter cranking it for about 30 seconds at a time. You should see some pressure on the oil pressure gauge although very low.
Once you are sure that the engine has been assembled correctly, then install the plugs and fire it up. If you still don't have any oil pressure, replace the oil pressure sending unit. Need a crows foot socket to get at the old one and to tighten up the new one. Snug is good.
If that is not it, let us know and we will scratch our head some more.
Cheers,
TQ