Can you apply a varnish over an oil based stain when using pine?

Yes you can. Varnish is oil based and compatible with oil based stains. Just make sure you give the stain ample time to dry. At least 24-48 hours.

I agree with William. I use oil-based stains all of the time (I work in pine about 99% of the time). After about 24 hours - I live in Arizona - I apply and oil based stand with excellent results.

Tung oil works very well and then you can use a water base polyacrylic over that for better protection

Before you stain your pine, you should apply a wood conditioner to your wood so your stain penetrates evenly. Then apply 1, 2 or 3 coats of stain let dry completely for a least 24 hours. Apply 2 to 3 coats of oil based varnish, sanding lightly between coats. Fully cures in about 1 week so be careful for the first week. Hope this helps.

If being used outside use a Marine Varnish!