Welcome,
The first thing you need to know is the width of the strap that your PO takes. That is the lug to lug width of your PO. Measure to the nearest millimeter the width between the two metal parts of the watch that holds the strap - these are the lugs of your watch and you need to measure the width between them.
IIRC, the lug width of the large size Planet Ocean is 22 mm. If you have the smaller midsize PO, it could be 20 mm.
Appreciate if anyone in the know could tell me whether AquaTerra leather straps could perfectly fit Planet Ocean?
It depends on the width of the strap (the width of the strap being the end that attaches to the watch). I can't say off-hand which model straps can fit another, we go by strap/lug width. Measure your PO lug width, and you should know the width size of the strap you need.
I bought a leather strap on ebay for my PO the other day, which fits my PO but it does not exactly fit like the original strap (there is a very small gap between the strap and the watch body)
It sounds as if you bought a strap that's too small for your watch (possibly a 20mm strap trying to fit onto a 22 mm space.) That's why it's loose.
Strap and lug width is the most basic measurement you need to know so that you can buy the correct strap to your watch.
I am sending you a PM with a few links that could help you.
Good luck and have fun.