What percentage of fuel mixture by O2 sensor?

p.s. either an approx range, or at cruise

The O2 sensor does not really directly control fuel mixture, the O2 signal is used to determine whether the fuel mixture needs to be trimmed. Primary fuel control is via the look up tables for any given RPM, air density/manifold pressure/air mass, temperature and throttle position (plus other inputs as deemed necessary by the engineers). Fuel trim percentages vary from make to make and application to application, one might see upwards of 20+% on a problem vehicle, anything over 10% is cause for suspicion.

I don't know that you can really answer that as you've phrased it. Basically, the O2 sensor input causes changes in the ECM's short-term and long-term fuel trim values so that the fuel mixture calculated from the trim values, as well as what the other sensors are saying, is at stoichiometric. The ECM will adjust the fuel mixture as much as necessary to keep the mixture at stoichiometric, until it reaches the limit of its adjustment and sets a trouble code. (There are some exceptions, like wide-open throttle on many vehicles, where the O2 sensor is ignored and power-enrichment settings are used.)