Are opamps as a voltage regulator effcient?
Opamps can't usually supply 100 mA.  But, they can still be used to control a power voltage if you add some current gain to their output.  If you really want a opamp to control a power voltage that can supply 100s of mA, here is a simple way:



The opamp still does the controlling and still provides voltage gain from Vin to Vout.  However, Q1 provides most of the Vout current.  R2 and R3 are the voltage feedback and the overall gain is the reciprocal of their attenuation ratio.  In this example R2 and R3 are equal, so the voltage gain from Vin to Vout is 2.

You might think all you need is the transistor and R2 and R3, but that could be unstable.  To make the opamp stable, you put a small cap directly between its output and inverting input.  But, you also need to let the opamp output not be loaded directly with the end load, especially if that load is capacitive.  R1 decouples the opamp output from the load such that the opamp should be able to reach whatever output voltage it wants to.  Without that, the stability feedback provided by C1 won't work right and the thing can oscillate anyway.

The best value of C1 is hard to predict, so start with something vaguely plausible, like I show, and then experiment.  Making it lower will invite instability and result in oscillations.  Making it higher will dampen the response to transient loads.  Test by connecting the worst case non-resistive load you expect, find the value of C1 just where it starts to be unstable, and then double it in the real circuit.

However, stepping back a couple of layers in this case, you don't need a opamp voltage regulator since the voltage you want is both a common value and known up front.  There are plenty of 3-terminal linear regulators specifically designed for this task that can source 100 mA.  If it's OK to waste the extra power as heat in the linear regulator, then this is clearly the way to go in your case.  In this case you're dropping 7.5V - 3.3V = 4.2V, which times 100 mA is 420 mW.  A linear regulator in a TO-220 or similar case will get warm, but won't require a heat sink.

If efficiency or getting rid of heat is a issue, then use a buck regulator to make the 3.3 V supply from the existing 7.5 V supply.