1. Try looking really closely at the one chasing, does he have any small, white bumps on the gill covers? That would show that he's a male and he's ready to breed
(It's possible that the other one is still a male, but that really wouldn't matter since it's the only fish around.. xD) Goldfish start chasing each other, nudging the lower bellies with their noses when they're in breeding mood
(Recently got two boys from the pet shop, pretty big and they started chasing my ladies around right away for days!)
2. How big is this tank and how long have you had it set up?
These two orandas can't be in anything smaller than a 20 gallon tank, a 30 would be ideal since orandas are the largest fancy goldfish there are! (They can grow to be a foot long if kept in proper conditions) So he might be uncomfortable because of the waste building up in case the tank is too small.
If your tank is 20 gallons, but you set it up less than 3 weeks ago, it's probably cycling, this means that the ammonia isn't being broken down by the beneficial bacteria yet, and that's extremely harmful to your fish! You'll need to have your water tested to see how high the ammonia levels are.
The only way to cycle a tank with fish is to change the water every time the tests tell you the waste levels are too high and add beneficial bacteria with every water change. (For example Stress Zyme by API or fluid filter media by EasyLife)
Here's another thread that explains cycling in detail: (Link is not visible to guests. Please register to view.
And here's one on how to maintain your tank: (Link is not visible to guests. Please register to view.