Can someone please explain what fare class (O) is on Delta?
Is this specific to any cabin class?

It's a business class award fare.

Thank you.
So it would be Delta One for transatlantic?

Yes. You didn't go through a broker and buy an award ticket, did you?

No, booked direct with Virgin, flight out of UK is Virgin UC but return with Delta currently shows Main Cabin (O).

O would be a main cabin fare if it is a VS coded revenue fare. Presuming this is an award ticket and the flight is DL coded.

Here is an old list. Mostly still good:

Fare Domestic Upg. Pecking Order
F First Class 1
J NA 1
C NA 2
P Discounted First 2
A Discounted First 3
D Discounted Business 3
G First Class 4
I Discounted Business 4
Z Discounted Business 5
B Yes 6
Y Yes* 6
M Yes 7
S Yes* 8
H Yes 9
Q Yes* 10
K Yes 11
L Yes* 12
U Yes* 13
T Yes* 14
X Yes* 15
V Yes* 16
E No 17
N See Notes NA
O NA NA
R Booked Award NA
W Premium NA
W+ Comfort+ NA

What does SW fare mean. Im sure it normal but just curious

S is no longer a revenue fare. It was recently moved to an award fare class. W is no longer used for Premium Economy fares. DL now uses P/A/G fare classes for Premium Economy (which are exclusively international), in addition to using them for discount domestic first class.

Does your ticket show a DL flight number or a VS (codeshare) flight number for the O segment? Can you see a seat map or pick a seat?

It reads to me that OP has VS O class on return, not DL O class. It is main cabin. He/she is asking in the wrong forum.

No, awards are booked as prime flight number. Thus since OP is on DL metal, he's booked on a DL flight number and in O class. This is Delta 1.

The VS system is confused and labels it Main cabin since O would be main cabin if booked as a VS flight.

The answer might depend on whether A class is available for the DL operated flight. Check EF or try a dummy booking (advanced search by fare class, A or higher on delta.dumb), although whether the FC cabin seems to be full will give some imperfect indication of whether DL is likely to be selling A tickets for the segments.

Fare classes are specific to marketing carrier, not operating carrier. Since it is AF marketed, no you are not entitled to FC on the flight.