OnePlus value sld hold firm in the immediate future(maybe 2-3months) mainly becuz of demand and availability. Demand high and availability low, seller can dictate the price. For Oppo's case, the demand is so-so only and probably due to the previous flash sales conducted by Lazada, at one point the market was flooded with brand new and used sets, driving down the selling price for the phone. The depreciation of value for Oppo phones is pretty drastic. Today you buy brand new for $700, next day you sell you make a least $100 loss. But say if you are talking about 1 year later, I think definitely no value even if in prestige condition since by then much better models will be available in the market.
Personally i feel that if the phone you receive has no issues whatsoever, unless you feel you need to make a small profit out of selling the phone, why not just keep it for personally use. Nowadays telco subsidies for contracted phones is getting pathetic unless you are on higher tier plans. Its really pointless getting bound by a 2 year contract just to "enjoy" a small discount upfront on phone prices. Not to mention telcos are regularly shifting the goalposts by imposing new charges for their services. I much rather buy off contract phone so that i am free of any contract obligations.