Is international Oppo Reno 12 compatible with mobile carriers in USA?

You can still buy the SIM.  If your device is not VoLTE or VoNR compatible.  The network will not allow you to activate the SIM in that device.  The US shut down 3G networks and 4G requires VoLTE.  California has fairly solid coverage.  T-Mobile does not block any device manufacture.  This is not Australia making a push to sell more carrier phones and lock out cheaper Asian imports.  Many Asian devices work fine on T-Mobile's network. It simply checks to see if the device is VoLTE compatible before SIM registration.