In a bowl, mix olive oil, lime juice, chili powder, cumin, and salt. Add chicken and marinate for 30 minutes.
Preheat grill to medium-high heat. Grill the chicken for 6-8 minutes per side until the internal temperature reaches 165 degrees F.
Remove chicken from grill, let it rest for 5 minutes, then chop into bite-sized pieces.
Warm the corn tortillas on the grill or a skillet for 30 seconds per side.
Assemble the tacos: place chopped chicken in tortillas, top with guacamole, diced white and red onions, fresh cilantro, and crumbled cheese. Serve immediately.