Q: transistor as a variable resistor for oscillator circuits
You could use a DDS which is easy (the hard work has been done for you). Assuming you don't want to do that, read on. 

For low frequencies (through the audio range) the easiest way is to generate triangle waves with a VCO which can then be shaped into sine waves if necessary. This can be done with a DAC to generate the control voltage and an oscillator made with an integrator and a comparator switching the integration direction. 

For high frequencies (1MHz and up) you can easily change the C by using a varactor diode. You can phase-lock the signal to a reference signal if you want. 

You can use a JFET or a lamp + LDR (light dependent resistor) to vary resistance, or as Ignacio says, use an MDAC to simulate a relatively low frequency resistor. 

Or you can use a digital pot! Some are specified well enough to use them as rheostats, though they are always better used as potentiometers. Again, not so great at high frequencies.