  We present the optical identification of mid-IR and radio sources detected in
the European Large Area ISO Survey (ELAIS) areas N1 and N2. Using the r' band
optical data from the Wide Field Survey we apply a likelihood ratio method to
search for the counterparts of the 1056 and 691 sources detected at 15 micron
and 1.4 GHz respectively, down to flux limits of S_{15}=0.5 mJy and S_{1.4
GHz}=0.135 mJy. We find that ~92% of the 15 micron ELAIS sources have an
optical counterpart down to the magnitude limit of the optical data, r'=24. All
mid-IR sources with fluxes S_{15} >= 3 mJy have an optical counterpart. The
magnitude distribution of the sources shows a well defined peak at relatively
bright magnitudes r'~18. About 20% of the identified sources show a point-like
morphology; its magnitude distribution has a peak at fainter magnitudes than
those of galaxies. The mid-IR-to-optical and radio-to-optical flux diagrams are
presented and discussed in terms of actual galaxy models. Objects with
mid-IR-to-optical fluxes larger than 1000 are found that can only be explained
as highly obscured star forming galaxies or AGNs. Blank fields being 8% of the
15 micron sample have even larger ratios suggesting that they may be associated
with higher redshift and higher obscured objects.
