No,the Find 7 is a GSM device and will not work on Sprints CDMA network. Same for Verizon. It'll work on AT & T and T Mobile though, and seems to have Verizon's LTE band,if you have good coverage with that.
OPPO sell their phones unsubsidized on their site and Amazon,no carrier agreements. Works great on AT&T strait talk unlimited.
Yup. There's plenty of math that supports an unsubsidized device on Straight Talk being FAR cheaper by the end of your 2-year contract than any subsidized device on AT&T.
In general, an AT&T contract subsidy is usually about $200-300 discount.
The cost premium of an AT&T plan over a Straight Talk one is usually on the order of $25/month
$25/month times 24 months (well, let's just say 18 since that's when most AT&T customers are eligible for early upgrade...): $450