Without the exact details of the train (where from? What's your destination, date, time of the train?), impossible to say what you do the best. 

Recommended is still optional. But if you want advice on what tot do, please specify route, date and departure time.

In this case “recommended” is there to indicate that the train is likely to be very busy and there may not be enough seats for all passengers.  You may buy a reservation and be guaranteed a seat or you may just turn up and get on the train (and hopefully get a seat without having to pay for a reservation) but you run the risk that there are no empty seats . . . in which case you get to stand for your entire trip.

“Required” means that you MUST buy a reservation to use this train.  You cannot travel on this train without having a reservation.

Mandatory is more used as required.

Rec. is what is says-but it is extremely very rare to have to stand all the way-trains make many stops and at any people get off and on. It may also mean you can sit on a seat UNres till next stop and then have to switch to another. People can be very creative in this and trains are not planes-you can easily get up and walk around. Quite often also seats labelled as RES will not be taken-after 10/15 mins anyone can then sit there. In these cases nearly always the seats have tiny screens which show from where to where they are RES (if it works properly).

For an overnite trip it is generally very unwise to NOT RES-even though in many cases that even is possible for the seats. This is mostly in DE and CZ.

There is no method by which the interrail app is in any position to predict the usage levels of any train, indications of reservation requirement are based on how the train type is listed, nothing more.

Some online resources can give indications of how busy some trains are likely to be based on historical data and/or current bookings, the DB app gives this for many of their own IC/ICE trains, this data, if available at all, tends to be only provided through operators own website/apps.

In order to be of any more specific help the actual train time and date is needed.

Details of the train

from London paddington to Oxford 

9:20-10:15

There is no method by which the interrail app is in any position to predict the usage levels of any train, indications of reservation requirement are based on how the train type is listed, nothing more.

I never said this was the app knew anything about usage levels.  A lot of discussions here seem to go off on tangents rather than discussing the original posters request.

London Paddington - Oxford has direct fast trains every half hour. There are also slightly slower trains every half hour from London Marylebone to Oxford via Bicester.

It is not a route I would bother reserving seats for but you can if you wish.

Any staffed railway station in UK can issue them or if you sign up to the website of www.gwr.com they have a seat reservation facility.

Both of these are free as are all seat reservations in Britain.