Which PSU do You have in it? (How many Watts)

assuming OP has 3080tower with stock psu which is 260w, you can upgrade psu to 500w for 7080/XPS8940 to support 2070.  The Dell 500w psu has two 4 pin cpu connectors. 3080T mobo has only one 4 pin cpu socket.  when upgrading to 500w psu just use one of the 4 pin connector.

optiplex-3080-spec-sheet.pdf (delltechnologies.com)

basic online psu calculator recommended psu for presumed spec: i5-10600 for 3080 tower, rtx 2070 is slightly less than 450w.  change cpu to i7-10700 makes no difference in psu estimate.

some practical points of fitting a discrete gpu in 3080.

although they look similar outside, 3080 tower is slightly smaller than 7080. most important size is depth of tower which determines max length of gpu.  3080 is about 0.3" smaller in depth than 7080.

3080: Front Height: 12.77 in. (324.30 mm)
Rear Height: 12.77 in. (324.30 mm)
Width: 6.06 in. (154.00 mm) Depth:
11.50 in. (292.20 mm)

7080: Front Height: 14.45 in.
(367.00 mm) Rear Height: 14.45 in.
(367.00 mm) Width: 6.65 in. (169.00
mm) Depth: 11.84 in. (300.80 mm)

it has been shown that 2070S can fit 7080 tower. Dell OptiPlex 7080 Performance Results - UserBenchmark

when choosing 2070 double check the card length can fit 3080.

Hello, apologies for the super late reply but I took a picture of my PSU

Photo is slightly blurry when expanded to see text.  But there's two 260w PSU's that are standard to the 3080.  Not too many options if you stick with the 260w PSU.

Optiplex 3080 Spec Sheet 

By the way, it looks like you have the tower and not the SFF?  That also makes a difference.