Ok, gaps can be filled by using the sand-dust of second sanding mixed with wood filler (various brands available e.g. Bona, Blanchon etc).
First sanding (40 grit) will get rid of old finish layer, dirt, grease etc. Empty dust bag of sander (and when doing this yourself, sanding that is, use a professional sander, not your regular drum sander of any hire company - will result in marks) and sand the floor with grit 80 sand paper. Use the sand dust to mix with filler and plaster the whole floor.
Let it dry and use grit 120 to sand for third time (removing all excess filler). If you want to lacquer the floor, sand a fourth time with grit 150, but I understand you want to oil it. Use any brand HardWaxOil. Easy to apply and loverly result. NB: any sanding errors (marks etc) will show once the finish layer is applied, hence the price for professional sanding.
How many square meters are we talking about?