I am attempting to do a simple OLS regression on time series data from 1990 to 2019, and there are trends in my variables: GDP per capita (dependent variable), oil prices, etc., which I can difference. I cannot difference variables such as inflation, as they are already stationary, as demonstrated on Eviews.

Is it acceptable in a model to have differenced variables and variables which are already stationary and if so, why? For context I am using log( gdp per capita) as my dependent variable, with oil price as my primary independent in combination with net exports and inflation to calculate the impact of oil price on a net exporting country.

I am unable to conduct (S)VAR analysis as I have not been taught this yet. Any further help would be greatly appreciated.