Wiki says first cotton mill 1783, 42 by 1800.
Most of those would have been small mills, employing about ten people according to Wiki?
The larger, especially coal/steam powered, mills were only just starting up at the turn of the century. 'Old Mill' in Ancoats was built in 1798, but would have been exceptional at the time, but small by mid to late 19th century standards.
I would think most of the weavers, and some spinners, who attended Peterloo would have been working in the domestic/cottage side of production. Both cotton and wool based
Looking at the marriage registers on the Lancashire OPC site might give some idea of probable range of occupations for those attending St Peters field in August 1819.
For example these occupations and places are taken from the marriage register of Saint Marys, in Oldham, January-March 1817. So yes, probably a lot of weavers there. Both industrial and domestic?
Spinner, [of] Shaw
Weaver, Mumps
Weaver, Hollinwood
Coalminer, Lowside
Weaver, Royton
Rover, King Street Oldham
Spinner, West Street Oldham
Farmer, Denton lane
Hatter, Manchester Street Oldham
Weaver, Royton
Weaver, Greenacres
Hatter, West Street Oldham
Weaver, Crompton
Weaver, Broadway lane
Coalminer, Crompton
Weaver, Glodwick
Weaver, Hunsworth
Cloather, White field
Weaver, George Street Oldham
Spinner, Chadderton
Weaver, Cannon Street Oldham
Weaver, Chadderton
Spinner, Bottom of Greenacres Moor
Weaver, Cowhill
Weaver, Hollinwood
Weaver, West Street Oldham