Hello, welcome to MFK.
Congrats on getting an Oscar, they are great fish. As far as keeping it with a turtle- It is risky. The turtle could take a chunk out of the Oscar. Oscars are generally pretty slow swimming, and at night they will sit still while they sleep. An opportunistic turtle could easily grab a bite, and the Oscar would most likely die from it's injuries. Also, a 75 is barely large enough for an Oscar alone, and turtles are very messy creatures, which also need the water level to be below the top of the tank (even less water).
So I would say no to the Oscar in the 75 with a turtle.
I would also advise you test your water frequently in the 55 as the fish grows, in 6 months your Oscar may be 8" long, and an 8" Oscar with two blood parrots in a 55 will make the water dirty very quickly.