Is it ok to mix gesso and oil paint before painting it on canvas?

Sure, if it's a modern gesso designed as a base for both oils and acrylics. But probably only in moderation as a tint. if you aren't sure of the chemical formula of the gesso, do a test patch first and let it dry thoroughly to see what happens.

If you're doing fine oils on canvas you might be interested to know that common Canola oil, the cheapest you can buy on sale at a discount market, is great for fine oil painting. Canola is rapeseed oil, a product of Canada (hence the name), and is as good as linseed oil, maybe even better. The old European masters used walnut oil. Just don't use any petroleum based oil because it won't dry.