It takes time to optimize the battery. Give it a week, and do this again. Sure vast difference. Each time there's a new android phone first one week all will scream the same thing. A week later all quiet once the batt is optimized. Your phone is fine, trust me

P/S : My friend bought same phone in US. She bought sat morning, now sunday night and battery still 21%.

Chill Tan Sri , I just dont expect it to be so lousy. Losing 10% on standby isn't in my standard for any phone and I never complained anything about the speed. Even my first android phone back in 2014 HTC m7 can last better than this
Anyway the previous oppo I used was the one offer by maxis,  a77 I believe and  xiaomi I got no idea but it's definitely the entry level but it's so good tbh.  I'm dont judge phone base on price tags and honestly I think it's on par with premium brands in terms of processing speed and battery life just not camera