Cat's don't cause or eliminate smoke.
I'd start with a compression test, checking the coolant for any murky discoloration or oily skin and check for chocolate pudding under the oil cap. Could be running excessively rich but that's more than a bad O2 sensor will generally cause. You're probably smoking while moving too, it's just getting thinned out and you don't see it.
Oh, and oil color means nothing.
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