Diverse types of neuron are interconnected via local or long-range synapses to form distinct neural circuits, which are basic modules for different brain functions and behaviors. Dissecting the circuit connectivity and function is one of fundamental studies to understand how the brain works. In the talk, I would present our recent findings on circuitry base for associative learning and cortical oscillations, by using electrophysiology and optogenetic approaches. The first part shows that in the entorhinal cortex (EC)→hippocampus CA1 direct circuit, lateral EC excitatory projection neurons selectively form direct synapses onto a specific population of morphologically complex, calbindin-expressing pyramidal cells in the dorsal CA1, forming a distinct circuit for olfactory associative learning. The second part will introduce some of our latest results of differential and cooperative roles of two major inhibitory interneurons, somatostatin (SOM)- and parvalbumin (PV)-expressing cells, in orchestrating diverse oscillatory activities in the visual cortical circuit.