The primer would accomplish that. It’s made to adhere to oil stained concrete then you apply the epoxy. Your method would work but isn’t needed with a proper primer.
If you want the absolute best finish, I believe that prep is 80% of that result! Primer will allow it to stick but if you want an even finish, you also need to grind uneven concrete, fill holes and cracks, smooth out rough edges, etc. but yes primer is a good option to help exist stick, always use primer. 
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I’ve seen a bunch of these DIY epoxy coated garage floors and DJPWS is correct. There generally isn’t a primer involved. The floor is acid etched after being degreased but before applying the 2-part epoxy. However, after talking with a professional concrete staining and coating contractor about my own garage floor and learning about all the prep work he does, I can see why the DIY jobs often look like crap after a few years. But for $3500 I decided to do SWISSTRAX instead.
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Exactly. And the better way to do it isn’t to acid etch it either, adds moisture and other chemical residues that can potentially ruin the finish. The best way is to grind it. it will leave a even finish but also leave it rough like acid etching would do without adding moisture or a chemical layer under your primer
Oil that has absorbed into concrete plays havoc with the adhesion of coatings which require penetration into substrate pores.
I would personally clean with degreaser and hot water multiple times to lift as much oil as possible, failing to do this prior to grinding will result in oil been ground into surface further and spread around everywhere.
Post grind I would test for oil contamination and degrease again if required followed by moisture testing prior to coating applications.
Selection of a high quality industrial concrete coating is vital, stay away from cheap coatings as they will inevitably fail.
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