Add heavy cream to a saucepan over medium heat, and bring to a simmer.
Add cheeses to pan and stir until completely melted (See Note 3).
Add green chiles, cumin and oregano. Stir thoroughly to combine. If too thick, thin with some more cream or milk to desired consistency.
Pour into serving dish and garnish with diced tomatoes, jalapenos and crumbled queso fresco, if desired. Serve warm with tortilla chips.
Video
Notes
If unable to find, you can substitute Oaxaca cheese with Asadero, Pepper Jack or Monterey Jack cheese. Both Oaxaca and Asadero are good quality Mexican cheeses that melt well.
Half & half and evaporated milk are good substitutes for heavy cream, if needed. However, the consistency of the final queso blanco dip will be a bit thinner.
Cheese separates at about 160°F which is why it curdles or separates if the sauce is pushing close to boiling. If it separates, off the heat source add some more cream (fat assists) and whisk until smooth and emulsified.